NOTE I.—METEORS.

THERE ſeem to be three concentric ſtrata of our incumbent atmoſphere; in which, or between them, are produced four kinds of meteors; lightning, ſhooting ſtars, fire-balls, and northern lights. Firſt, the lower region of air, or that which is denſe enough to reſiſt by the adheſion of its particles the deſcent of condenſed vapour, or clouds, which may extend from one to three or four miles high. In this region the common lightning is produced from the accumulation or defect of electric matter in thoſe floating fields of vapour either in reſpect to each other, or in reſpect to the earth beneath them, or the diſſolved vapour above them, which is conſtantly varying both with the change of the form of the clouds, which thus evolve a greater or leſs ſurface; and alſo with their ever-changing degree of condenſation. As the lightning is thus produced in denſe air, it proceeds but a ſhort courſe on account of the greater reſiſtance which it encounters, is attended with a loud exploſion, and appears with a red light.

2. The ſecond region of the atmoſphere I ſuppoſe to be that which has too little tenacity to ſupport condenſed vapour or clouds; but which yet contains inviſible vapour, or water in aerial ſolution. This aerial ſolution of water differs from that diſſolved in the matter of heat, as it is supported by its adheſion to the particles of air, and is not pre⯑cipitated by cold. In this ſtratum it ſeems probable that the meteors called ſhooting ſtars are produced; and that they conſiſt of electric ſparks, or lightning, paſſing from one region to another of theſe inviſible fields of aero-aqueous ſolution. The height of theſe ſhooting ſtars has not yet been aſcertained by ſufficient obſervation; Dr. Blagden thinks their ſituation is lower down in the atmoſphere than that of fireballs, which he conjectures from their ſwift apparent motion, and aſcribes their ſmallneſs to the more minute diviſion of the electric matter of which they are suppoſed to conſiſt, owing to [2] the geater reſiſtance of the denſer medium through which they paſs, than that in which the fire-balls exiſt. Mr. Brydone obſerved that the shooting stars appeared to him to be as high in the atmoſphere, when he was near the summit of mount Etna, as they do when obſerved from the plain. Phil. Tran. Vol. LXIII.

As the stratum of air, in which shooting stars are suppoſed to exiſt is much rarer than that in which lightning reſides, and yet much denſer than that in which fire⯑balls are produced, they will be attracted at a greater diſtance than the former, and at a leſs than the latter. From this rarity of the air so small a sound will be pro⯑duced by their exploſion, as not to reach the lower parts of the atmoſphere; their quantity of light from their greater diſtance being small, is never seen through denſe air at all, and thence does not appear red, like lightning or fire balls. There are no apparent clouds to emit or to attract them, becauſe the conſtituent parts of theſe aero-aqueous regions may poſſeſs an abundance or deficiency of electric matter and yet be in perfect reciprocal solution. And laſtly their apparent train of light is probably owing only to a continuance of their impreſſion on the eye; as when a fire-stick is whirled in the dark it gives the appearance of a compleat circle of fire: for theſe white trains of shooting stars quickly vaniſh, and do not seem to set any thing on fire in their paſſage, as seems to happen in the tranſit of fire-balls.

3. The second region or stratum of air terminates I suppoſe where the twilight ceaſes to be refracted, that is, where the air is 3000 times rarer than at the surface of the earth; and where it seems probable that the common air ends, and is ſurrounded by an atmo⯑ſphere of inflammable gas tenfold rarer than itſelf. In this region I believe fire-balls ſometimes to paſs, and at other times the northern lights to exiſt. One of theſe fire-balls or draco volans, was obſerved by Dr. Pringle and many others on Nov. 26, 1758, which was afterwards eſtimated to have been a mile and a half in circumference, to have been about one hundred miles high, and to have moved towards the north with a velocity of near thirty miles in a ſecond of time. This meteor had a real tail many miles long, which threw off ſparks in its courſe, and the whole exploded with a sound like diſtant thunder. Philoſ. Tranſ. Vol. LI.

Dr. Blagden has related the hiſtory of another large meteor, or fire-ball, which was ſeen the 18th of Auguſt, 1783, with many ingenious obſervations and conjectures. This was eſtimated to be between 60 and 70 miles high, and to travel 1000 miles at the rate of about twenty miles in a ſecond. This fire-ball had likewiſe a real train of light left behind it in its paſſage, which varied in colour; and in ſome part of its courſe gave off ſparks or exploſions where it had been brighteſt; and a duſky red streak remained viſible perhaps a minute. Philoſ. Tranſ. Vol. LXXIV.

Theſe fire-balls differ from lightning, and from ſhooting ſtars in many remarkable circumſtances; as their very great bulk, being a mile and a half in diameter; their tra⯑velling 1000 miles nearly horizontally; their throwing off sparks in their paſſage; and changing colours from bright blue to duſky red; and leaving a train of fire behind them, continuing about a minute. They differ from the northern lights in not being diffuſed, but paſſing from one point of the heavens to another in a defined line; and this in a region above the crepuſcular atmoſphere, where the air is 3000 times rarer than at the [3] ſurface of the earth. There has not yet been even a conjecture which can account for theſe appearances!—One I ſhall therefore hazard; which, if it does not inform, may amuſe the reader.

In the note on l. 123, it was ſhewn that there is probably a ſupernatant ſtratum of inflammable gas or hydrogene, over the common atmoſphere; and whoſe denſity at the ſurface where they meet, muſt be at leaſt ten times leſs than that upon which it ſwims; like chemical ether floating upon water, and perhaps without any real contact. 1. In this region, where the aerial atmoſphere terminates and the inflammable one begins, the quantity of tenacity or reſiſtance muſt be almoſt inconceivable; in which a ball of elec⯑tricity might paſs 1000 miles with greater eaſe than through a thouſandth part of an inch of glaſs. 2. Such a ball of electricity paſſing between inflammable and common air would ſet fire to them in a line as it paſſed along; which would differ in colour accord⯑ing to the greater proportionate commixture of the two airs; and from the same cauſe there might occur greater degrees of inflammation, or branches of fire, in ſome parts of its courſe.

As theſe fire-balls travel in a defined line, it is pretty evident from the known laws of electricity, that they muſt be attracted; and as they are a mile or more in diameter, they muſt be emitted from a large ſurface of electric matter; becauſe large nobs give larger ſparks, leſs diffuſed, and more brightly luminous, than leſs ones or points, and reſiſt more forceably the emiſſion of the electric matter. What is there in nature can attract them at ſo great a diſtance as 1000 miles, and so forceably as to detach an electric ſpark of a mile diameter? Can volcanos at the time of their eruptions have this effect, as they are generally attended with lightning? Future obſervations muſt diſcover theſe ſecret operations of nature! As a stream of common air is carried along with the paſſage of electric aura from one body to another; it is eaſy to conceive, that the common air and the inflammable air between which the fire-ball is ſuppoſed to paſs, will be partially intermixed by being thus agitated, and ſo far as it becomes intermixed it will take fire, and produce the linear flame and branching ſparks above deſcribed. In this circumſtance of their being attracted, and thence paſſing in a defined line, the fire-balls ſeem to differ from the coruſcations of the aurora borealis, or northern lights, which probably take place in the ſame region of the atmoſphere; where the common air exiſts in extreme tenuity, and is covered by a ſtill rarer ſphere of inflammable gas, ten times lighter than itſelf.

As the electric ſtreams, which conſtitute theſe northern lights, ſeem to be repelled or radiated from an accumulation of that fluid in the north, and not attracted like the fire⯑balls; this accounts for the diffuſion of their light, as well as the ſilence of their paſſage; while their variety of colours, and the permanency of them, and even the breadth of them in different places, may depend on their ſetting on fire the mixture of inflammable and common air through which they paſs; as ſeems to happen in the tranſit of the fire-balls.

[4] It was obſerved by Dr. Prieſtley that the electric ſhock taken through inflammable air was red, in common air it is blueiſh; to theſe circumſtances perhaps ſome of the colours of the northern lights may bear analogy; though the denſity of the medium through which light is ſeen muſt principally vary its colour, as is well explained by Mr. Morgan. Phil. Tranſ. Vol. LXXV. Hence lightning is red when ſeen through a dark cloud, or near the horizon; becauſe the more refrangible rays cannot permeate so denſe a medium. But the ſhooting ſtars conſiſt of white light, as they are generally ſeen on clear nights, and nearly vertical: in other ſituations their light is probably too faint to come to us. But as in ſome remarkable appearances of the northern lights, as in March, 1716, all the priſmatic colours were ſeen quickly to ſucceed each other, theſe appear to have been owing to real combuſtion; as the denſity of the interpoſed medium could not be ſuppoſed to change ſo frequently; and therefore theſe colours muſt have been owing to different degrees of heat according to Mr. Morgan's theory of combuſtion. In Smith's Optics, p. 69. the priſmatic colours, and optical deceptions of the northern lights are deſcribed by Mr. Cotes.

The Torricellian vacuum, if perfectly free from air, is ſaid by Mr. Morgan and others to be a perfect non-conductor. This circumſtance therefore would preclude the electric ſtreams from riſing above the atmoſphere. But as Mr. Morgan did not try to paſs an electric ſhock through a vacuum, and as air, or ſomething containing air, sur⯑rounding the tranſit of electricity may be neceſſary to the production of light, the con⯑cluſion may perhaps still be dubious. If however the streams of the northern lights were suppoſed to riſe above our atmoſphere, they would only be viſible at each extremity of their courſe; where they emerge from, or are again immerged into the atmoſphere; but not in their journey through the vacuum; for the abſence of electric light in a vacuum is ſufficiently proved by the common experiment of ſhaking a barometer in the dark; the electricity, produced by the friction of the mercury in the glaſs at its top, is luminous if the barometer has a little air in it; but there is no light if the vacuum be complete.

The aurora borealis, or northern dawn, is very ingeniouſly accounted for by Dr. Franklin on principles of electricity. He premiſes the following electric phenomena: 1. that all new fallen snow has much poſitive electricity standing on its ſurface. 2. That about twelve degrees of latitude round the poles are covered with a cruſt of eternal ice, which is impervious to the electric fluid. 3. That the denſe part of the atmoſphere riſes but a few miles high; and that in the rarer parts of it the electric fluid will paſs to almoſt any diſtance.

Hence he suppoſes there muſt be a great accumulation of poſitive electric matter on the freſh fallen ſnow in the polar regions; which, not being able to paſs through the cruſt of ice into the earth, muſt riſe into the rare air of the upper parts of our atmoſphere, which will the leaſt reſiſt its paſſage; and paſſing towards the equator deſcend again into the denſer atmoſphere, and thence into the earth in silent streams. And that many of the appearances attending theſe lights are optical deceptions, owing to the situation of the eye that beholds them; which makes all aſcending parallel lines appear to converge to a point.

[5] The idea, above explained in note on l. 123, of the exiſtence of a sphere of inflam⯑mable gas over the aerial atmoſphere would much favour this theory of Dr. Franklin; becauſe in that caſe the denſe aerial atmoſphere would riſe a much leſs height in the polar regions, diminiſhing almoſt to nothing at the pole itſelf; and thus give an eaſier paſſage to the aſcent of the electric fluid. And from the great difference in the ſpecific gravity of the two airs, and the velocity of the earth's rotation, there muſt be a place between the poles and the equator, where the superior atmoſphere of inflammable gas would terminate; which would account for theſe ſtreams of the aurora borealis not appearing near the equator; add to this that it is probable the electric fluid may be heavier than the magnetic one; and will thence by the rotation of the earth's surface aſcend over the magnetic one by its centrifugal force; and may thus be induced to riſe through the thin ſtratum of aerial atmoſphere over the poles. See note on Canto II. l. 193. I ſhall have occaſion again to mention this great accumulation of inflammable air over the poles; and to conjecture that theſe northern lights may be pro⯑duced by the union of inflammable with common air, without the aſſiſtance of the electric ſpark to throw them into combuſtion.

The antiquity of the appearance of northern lights has been doubted; as none were recorded in our annals ſince the remarkable one on Nov. 14, 1574, till another remark⯑able one on March 6, 1716, and the three following nights, which were ſeen at the same time in Ireland, Ruſſia, and Poland, extending near 30 degrees of longitude and from about the 50th degree of latitude over almoſt all the north of Europe. There is however reaſon to believe them of remote antiquity though inaccurately deſcribed; thus the following curious paſſage from the Book of Maccabees, (B. II. c. v.) is ſuch a de⯑ſcription of them, as might probably be given by an ignorant and alarmed people. ‘Through all the city, for the ſpace of almoſt forty days, there were ſeen horſemen run⯑ning in the air, in cloth of gold, and armed with lances, like a band of ſoldiers; and troops of horſemen in array encountering and running one againſt another, with ſhak⯑ing of ſhields and multitude of pikes, and drawing of ſwords, and caſting of darts, and glittering of golden ornaments and harneſs.’

NOTE II.—PRIMARY COLOURS.

THE manner in which the rainbow is produced was in some meaſure underſtood before Sir Iſaac Newton had diſcovered his theory of colours. The firſt perſon who expreſſly ſhewed the rainbow to be formed by the reflection of the sunbeams from drops of falling rain was Antonio de Dominis. This was afterwards more fully and diſtinctly explained by Des Cartes. But what cauſed the diverſity of its colours was not then un⯑derſtood; it was reſerved for the immortal Newton to diſcover that the rays of light con⯑siſted of ſeven combined colours of different refrangibility, which could be ſeperated at pleaſure by a wedge of glaſs. Pemberton's View of Newton.

Sir Iſaac Newton diſcovered that the priſmatic ſpectrum was compoſed of ſeven colours in the following proportions, violet 80, indigo 40, blue 60, green 60, yellow 48, orange 27, red 45. If all theſe colours be painted on a circular card in the proportions above men⯑tioned, and the card be rapidly whirled on its center, they produce in the eye the ſenſa⯑tion of white. And any one of theſe colours may be imitated by painting a card with the two colours which are contiguous to it, in the ſame proportions as in the ſpectrum, and whirling them in the ſame manner.

My ingenious friend, Mr. Galton of Birmingham, aſcertained in this manner by a set of experiments the following propoſitions; the truth of which he had preconceived from the above data.

1. Any colour in the priſmatic ſpectrum may be imitated by a mixture of the two colours contiguous to it.

2. If any three succeſſive colours in the priſmatic ſpectrum are mixed, they compoſe only the ſecond or middlemoſt colour.

3. If any four succeſſive colours in the priſmatic ſpectrum be mixed, a tint similar to a mixture of the ſecond and third colours will be produced, but not preciſely the ſame, becauſe they are not in the ſame proportion.

4. If beginning with any colour in the circular ſpectrum, you take of the ſecond colour a quantity equal to the firſt, second, and third; and add to that the fifth colour, equal in quantity to the fourth, fifth, and ſixth; and with theſe combine the ſeventh colour in the proportion it exiſts in the ſpectrum, white will be produced. Becauſe the firſt, ſecond, and third, compoſe only the ſecond; and the fourth, fifth, and ſixth, com⯑poſe only the fifth; therefore if the ſeventh be added, the ſame effect is produced, as if all the ſeven were employed.

5. Beginning with any colour in the circular ſpectrum, if you take a tint compoſed of a certain proportion of the ſecond and third, (equal in quantity to the firſt, ſecond, third, and fourth,) and add to this the ſixth colour equal in quantity to the fifth, ſixth, and ſeventh, white will be produced.

[7] From theſe curious experiments of Mr. Galton many phenomena in the chemical changes of colours may probably become better underſtood; eſpecially if, as I ſuppoſe, the ſame theory muſt apply to tranſmitted colours, as to reflected ones. Thus it is well known, that if the glaſs of mangoneſe, which is a tint probably compoſed of violet and indigo, be mixed in a certain proportion with the glaſs of lead, which is yellow; that the mixture becomes tranſparent. Now from Mr. Galton's experiments it appears, that in reflected colours such a mixture would produce white, that is, the same as if all the colours were reflected. And therefore in tranſmitted colours the ſame circumſtances muſt pro⯑duce tranſparency, that is, the same as if all the colours were tranſmitted. For the particles, which conſtitute the glaſs of mangoneſe will tranſmit red, violet, indigo, and blue; and thoſe of the glaſs of lead will tranſmit orange, yellow, and green; hence all the primary colours by a mixture of theſe glaſſes become tranſmitted, that is, the glaſs becomes tranſparent.

Mr. Galton has further obſerved that five succeſſive priſmatic colours may be com⯑bined in ſuch proportions as to produce but one colour, a circumſtance which might be of conſequence in the art of painting. For if you begin at any part of the circular spectrum above deſcribed, and take the firſt, second, and third colours in the proportions in which they exiſt in the ſpectrum; theſe will compoſe only the ſecond colour equal in quantity to the firſt, ſecond, and third; add to theſe the third, fourth, and fifth in the proportion they exiſt in the ſpectrum, and theſe will produce the fourth colour equal in quantity to the third, fourth, and fifth. Conſequently this is preciſely the ſame thing, as mixing the ſecond and fourth colours only; which mixture would only produce the third colour. Therefore if you combine the firſt, ſecond, fourth, and fifth in the proportions in which they exiſt in the ſpectrum, with double the quantity of the third colour, this third colour will be produced. It is probable that many of the unexpected changes in mixing colours on a painter's eaſle, as well as in more fluid chemical mixtures, may depend on theſe principles rather than on a new arrangement or combination of their minute particles.

Mr. Galton further obſerves, that white may univerſally be produced by the com⯑bination of one priſmatic colour, and a tint intermediate to two others. Which tint may be diſtinguiſhed by a name compounded of the two colours, to which it is intermediate. Thus white is produced by a mixture of red with blue-green. Of orange with indigo⯑blue. Of Yellow with violet-indigo. Of green with red-violet. Of blue with Orange-red. Of indigo with yellow-orange. Of violet with green-yellow. Which he further remarks exactly coincides with the theory and facts mentioned by Dr. Robert Darwin of Shrewſ⯑bury in his account of ocular ſpectra; who has ſhewn that when one of theſe contraſted colours has been long viewed, a ſpectrum or appearance of the other becomes viſible in the fatigued eye. Philoſ. Tranſ. Vol. LXXVI. for the year 1786.

Theſe experiments of Mr. Galton might much aſſiſt the copper-plate printers of callicoes and papers in colours; as three colours or more might be produced by two copper-plates. Thus ſuppoſe ſome yellow figures were put on by the firſt plate, and upon ſome parts of theſe yellow figures and on other parts of the ground blue was laid on by another copper-plate. The three colours of yellow, blue, and green might be produced; as green leaves with yellow and blue flowers.

NOTE III.—COLOURED CLOUDS.

THE rays from the riſing and ſetting ſun are refracted by our ſpherical atmoſphere, hence the moſt refrangible rays, as the violet, indigo, and blue are reflected in greater quantities from the morning and evening ſkies; and the leaſt refrangible ones, as red and orange, are laſt ſeen about the ſetting ſun. Hence Mr. Beguelin obſerved that the ſhadow of his finger on his pocket-book was much bluer in the morning and evening, when the ſhadow was about eight times as long as the body from which it was projected. Mr. Melville obſerves, that the blue rays being more refrangible are bent down in the evenings by our atmoſphere, while the red and orange being leſs refrangible continue to paſs on and tinge the morning and evening clouds with their colours. See Prieſtley's Hiſtory of Light and Colours, p. 440. But as the particles of air, like thoſe of water, are themſelves blue, a blue ſhadow may be ſeen at all times of the day, though much more beautifully in the mornings and evenings, or by means of a candle in the middle of the day. For if a ſhadow on a piece of white paper is produced by placing your finger between the paper and a candle in the day light, the ſhadow will appear very blue; the yellow light of the candle upon the other parts of the paper apparently deepens the blue by its contraſt; theſe colours being oppoſite to each other, as explained in note II.

Colours are produced from clouds or miſts by refraction, as well as by reflection. In riding in the night over an unequal country I obſerved a very beautiful coloured halo round the moon, whenever I was covered with a few feet of miſt, as I aſcended from the vallies; which ceaſed to appear when I roſe above the miſt. This I ſuppoſe was owing to the thinneſs of the stratum of miſt, in which I was immerſed; had it been thicker, the colours refracted by the ſmall drops, of which a fog conſiſts, would not have paſſed through it down to my eye.

There is a bright ſpot seen on the cornea of the eye, when we face a window, which is much attended to by portrait painters; this is the light reflected from the ſpherical surface of the poliſhed cornea, and brought to a focus; if the obſerver is placed in this focus, he ſees the image of the window; if he is placed before or behind the focus, he only sees a luminous ſpot, which is more luminous and of leſs extent, the nearer he approaches to the focus. The luminous appearance of the eyes of animals in the duſky corners of a room, or in holes in the earth, may ariſe in ſome inſtances from the same principle; viz. the reflection of the light from the spherical cornea; which will be coloured red or blue in ſome degree by the morning, evening, or meridian light; or by the objects from which that light is previouſly reflected. In the cavern at Colebrook Dale, where the mineral tar exſudes, the eyes of the horſe, which was drawing a cart from within [9] towards the mouth of it, appeared like two balls of phoſphorus, when he was above 100 yards off, and for a long time before any other part of the animal was viſible. In this caſe I suſpect the luminous appearance to have been owing to the light, which had entered the eye, being reflected from the back surface of the vitreous humour, and thence emerg⯑ing again in parallel rays from the animals eye, as it does from the back surface of the drops of the rainbow, and from the water-drops which lie, perhaps without contact, on cabbage leaves, and have the brilliancy of quickſilver. This accounts for this luminous appearance being beſt seen in thoſe animals which have large apertures in their iris, as in cats and horſes, and is the only part viſible in obſcure places, becauſe this is a better re⯑flecting surface than any other part of the animal. If any of theſe emergent rays from the animals eye can be suppoſed to have been reflected from the choroid coat through the semi-tranſparent retina, this would account for the coloured glare of the eyes of dogs or cats and rabits in dark corners.

NOTE IV.—COMETS.

THERE have been many theories invented to account for the tails of comets. Sir Iſaac Newton thinks that they conſiſt of rare vapours raiſed from the nucleus of the comet, and ſo rarefied by the sun's heat as to have their general gravitation diminiſhed, and that they in conſequence aſcend oppoſite to the ſun, and from thence reflect the rays of light. Dr. Halley compares the light of the tails of comets to the ſtreams of the aurora berealis, and other electric effluvia. Philoſ. Tranſ. No. 347.

Dr. Hamilton obſerves that the light of ſmall ſtars are ſeen undiminiſhed through both the light of the tails of comets, and of the aurora borealis, and has further illuſtrated their electric analogy, and adds that the tails of comets conſiſt of a lucid ſelf-ſhining ſub⯑ſtance which has not the power of refracting or reflecting the rays of light. Eſſays.

The tail of the comet of 1744 at one time appeared to extend above 16 degrees from its body, and muſt have thence been above twenty three millions of miles long. And the comet of 1680, according to the calculations of Dr. Halley on November the 11th, was not above one ſemi-diameter of the earth, or leſs than 4000 miles to the northward of the way of the earth; at which time had the earth been in that part of its orbit, what might have been the conſequence! no one would probably have ſurvived to have regiſtered the tremendous effects.

[10] The comet of 1531, 1607, and 1682 having returned in the year 1759, according to Dr. Halley's prediction in the Philoſ. Tranſ. for 1705, there ſeems no reaſon to doubt that all the other comets will return after their proper periods. Aſtronomers have in general acquieſced in the conjecture of Dr. Halley, that the comets of 1532, and 1661 are one and the ſame comet, from the ſimilarity of the elements of their orbits, and were therefore induced to expect its return to its perihelium 1789. As this comet is liable to be diſturbed in its aſcent from the ſun by the planets Jupiter and Saturn, Dr. Maſkelyne expected its return to its perihelium in the beginning of the year 1789, or the latter end of the year 1788, and certainly ſometime before the 27th of April, 1789, which pre⯑diction has not been fulfilled. Phil. Trans. Vol. LXXVI.

NOTE V.—SUN'S RAYS.

THE diſpute among philoſophers about phlogiſton is not concerning the exiſtence of an inflammable principle, but rather whether there be one or more inflammable principles. The diſciples of Stahl, which till lately included the whole chemical world, believed in the identity of phlogiſton in all bodies which would flame or calcine. The diſciples of Lavoiſier pay homage to a plurality of phlogiſtons under the various names of charcoal, sulphur, metals, &c. Whatever will unite with pure air, and thence compoſe an acid, is eſteemed in this ingenious theory to be a different kind of phlogiſtic or inflammable body. At the same time there remains a doubt whether theſe inflammable bodies, as metals, ſulphur, charcoal, &c. may not be compounded of the ſame phlogiſton along with ſome other material yet undiſcovered, and thus an unity of phlogiſton exiſt, as in the theory of Stahl, though very differently applied in the explication of chemical phenomena.

Some modern philoſophers are of opinion that the ſun is the great fountain from which the earth and other planets derive all the phlogiſton which they poſſes; and that this is formed by the combination of the ſolar rays with all opake bodies, but particularly with the leaves of vegetables, which they suppoſe to be organs adapted to abſorb them. And that as animals receive their nouriſhment from vegetables they alſo obtain in a ſecondary manner their phlogiſton from the ſun. And laſtly as great maſſes of the mineral kingdom, which have been found in the thin cruſt of the earth which human labour has penetrated, have evidently been formed from the recrements of animal and vegetable bodies, theſe alſo are suppoſed thus to have derived their phlogiſton from the sun.

Another opinion concerning the ſun's rays is, that they are not luminous till they arrive at our atmoſphere; and that there uniting with ſome part of the air they produce [11] combuſtion, and light is emitted, and that an etherial acid, yet undiſcovered, is formed from this combuſtion.

The more probable opinion is perhaps, that the ſun is a phlogiſtic maſs of matter, whoſe ſurface is in a ſtate of combuſtion, which like other burning bodies emits light with immenſe velocity in all directions; that theſe rays of light act upon all opake bodies, and combining with them either diſplace or produce their elementary heat, and become chemically combined with the phlogiſtic part of them; for light is given out when phlogiſtic bodies unite with the oxygenous principle of the air, as in combuſtion, or in the reduction of metallic calxes; thus in preſenting to the flame of a candle a letter⯑wafer, (if it be coloured with red-lead,) at the time the red-lead becomes a metallic drop, a flaſh of light is perceived. Dr. Alexander Wilſon very ingeniouſly endeavours to prove that the sun is only in a ſtate of combuſtion on its ſurface, and that the dark ſpots ſeen on the diſk are excavations or caverns through the luminous cruſt, some of which are 4000 miles in diameter. Phil. Trans. 1774. Of this I ſhall have occaſion to speak again.

NOTE VI.—CENTRAL FIRES.

M. DE MAIRAN in a paper publiſhed in the Hiſtoire de l'Academic de Sciences, 1765, has endeavoured to ſhew that the earth receives but a ſmall part of the heat which it poſſeſſes, from the ſun's rays, but is principally heated by fires within itſelf. He thinks the ſun is the cauſe of the viciſſitudes of our ſeaſons of ſummer and winter by a very ſmall quantity of heat in addition to that already reſiding in the earth, which by emana⯑tions from the centre to the circumference renders the ſurface habitable, and without which, though the ſun was conſtantly to illuminate two thirds of the globe at once, with a heat equal to that at the equator, it would ſoon become a maſs of ſolid ice. His reaſonings and calculations on this ſubject are too long and too intricate to be in⯑serted here, but are equally curious and ingenious and carry much conviction along with them.

The opinion that the center of the earth conſiſts of a large maſs of burning lava, has been eſpouſed by Boyle, Boerhave, and many other philoſophers. Some of whom con⯑ſidering its ſuppoſed effects on vegetation and the formation of minerals have called it a ſecond ſun. There are many arguments in ſupport of this opinion. 1. Becauſe the power of the ſun does not extend much beyond ten feet deep into the earth, all below being in winter and ſummer always of the ſame degree of heat, viz. 48, which being [12] much warmer than the mildeſt froſt, is ſuppoſed to be ſuſtained by ſome internal diſtant fire. Add to this however that from experiments made ſome years ago by Dr. Franklin the ſpring-water at Philadelphia appeared to be of 52° of heat, which ſeems further to confirm this opinion, ſince the climates in North America are ſuppoſed to be colder than thoſe of Europe under ſimilar degrees of latitude. 2. Mr. De Luc in going 1359 feet perpendicular into the mines of Hartz on July the 5th, 1778, on a very fine day found the air at the bottom a little warmer than at the top of the ſhaft. Phil. Tranſ. Vol. LXIX. p. 488. In the mines in Hungary, which are 500 cubits deep, the heat becomes very troubleſome when the miners get below 480 feet depth. Morinus de Locis ſubter. p. 131. But as ſome other deep mines as mentioned by Mr. Kirwan are ſaid to poſſeſs but the common heat of the earth; and as the cruſt of the globe thus penetrated by human labour is ſo thin compared with the whole, no certain deduction can be made from theſe facts on either ſide of the queſtion. 3. The warm-ſprings in many parts of the earth at great diſtance from any Volcanos ſeem to originate from the condenſation of vapours ariſing from water which is boiled by ſubterraneous fires, and cooled again in their pas⯑ſage through a certain length of the colder ſoil; for the theory of chemical ſolution will not explain the equality of their heat at all ſeaſons and through ſo many centuries. See note on Fucus in Vol. II. See a letter on this ſubject in Mr. Pilkinton's View of Derby⯑ſhire from Dr. Darwin. 4. From the ſituations of volcanos which are always found upon the ſummit of the higheſt mountains. For as theſe mountains have been lifted up and loſe ſeveral of their uppermoſt ſtrata as they riſe, the loweſt ſtrata of the earth yet known appear at the tops of the higheſt hills; and the beds of the Volcanos upon theſe hills muſt in conſequence belong to the loweſt ſtrata of the earth, conſiſting perhaps of granite or baſaltes, which were produced before the exiſtance of animal or vegetable bodies, and might conſtitute the original nucleus of the earth, which I have ſuppoſed to have been projected from the ſun, hence the volcanos themſelves appear to be ſpira⯑cula or chimneys belonging to great central fires. It is probably owing to the eſcape of the elaſtic vapours from theſe ſpiracula that the modern earthquakes are of ſuch ſmall ex⯑tent compared with thoſe of remote antiquity, of which the veſtiges remain all over the globe. 5. The great ſize and height of the continents, and the great ſize and depth of the South-ſea, Atlantic, and other oceans, evince that the firſt earthquakes, which pro⯑duced theſe immenſe changes in the globe, muſt have been occaſioned by central fires. 6. The very diſtant and expeditious communication of the ſhocks of ſome great earth⯑quakes. The earthquake at Liſbon in 1755 was perceived in Scotland, in the Peak of Derbyſhire, and in many other diſtant parts of Europe. The percuſſions of it travelled with about the velocity of ſound, viz. about thirteen miles in a minute. The earthquake in 1693 extended 2600 leagues. (Goldſmith's Hiſtory.) Theſe phenomena are eaſily ex⯑plained if the central parts of the earth conſiſt of a fluid lava, as a percuſſion on one part of ſuch a fluid maſs would be felt on other parts of its confining vault, like a ſtroke on a fluid contained in a bladder, which however gentle on one ſide is perceptible to the hand placed on the other; and the velocity with which ſuch a concuſſion would travel would be that of ſound, or thirteen miles in a minute. For further information on this part of the ſubject the reader is referred to Mr. Michell's excellent Treatiſe on Earthquakes in the [13] Philos. Tranſ. Vol. LI. 7. That there is a cavity at the center of the earth is made pro⯑bable by the late experiments on the attraction of mountains by Mr. Maſkerlyne, who ſup⯑poſed from other conſiderations that the denſity of the earth near the ſurface ſhould be five times leſs than its mean denſity. Phil. Trans. Vol. LXV. p. 498. But found from the attraction of the mountain Schehallien, that it is probable, the mean denſity of the earth is but double that of the hill. Ibid. p. 532. Hence if the firſt ſuppoſition be well founded there would appear to be a cavity at the centre of conſiderable magnitude, from whence the immenſe beds and mountains of lava, toadſtone, baſaltes, granite, &c. have been protruded. 8. The variation of the compaſs can only be accounted for by ſup⯑poſing the central parts of the earth to conſiſt of a fluid maſs, and that part of this fluid is iron, which requiring a greater degree of heat to bring it into fuſion than glaſs or other metals, remains a ſolid, and the vis inertiae of this fluid maſs with the iron in it, occaſions it to perform ſewer revolutions than the cruſt of ſolid earth over it, and thus it is gradu⯑ally left behind, and the place where the floating iron reſides is pointed to by the direct or retrograde motions of the magnetic needle. This ſeems to have been nearly the opinion of Dr. Halley and Mr. Euler.

NOTE VII.—ELEMENTARY HEAT.

A CERTAIN quantity of heat ſeems to be combined with all bodies beſides the ſenſible quantity which gravitates like the electric fluid amongſt them. This combined heat or latent heat of Dr. Black, when ſet at liberty by fermentation, inflammation, cryſtallization, freezing, or other chemical attractions producing new combinations, paſſes as a fluid element into the ſurrounding bodies. And by thawing, diffuſion of neutral ſalts in water, melting, and other chemical ſolutions, a portion of heat is attracted from the bodies in vicinity and enters into or becomes combined with the new ſolutions.

Hence a combination of metals with acids, of eſſential oils and acids, of alcohol and water, of acids and water, give out heat; whilſt a ſolution of ſnow in water or in acids, and of neutral ſalts in water, attract heat from the ſurrounding bodies. So the acid of nitre mixed with oil of cloves unites with it and produces a moſt violent flame; the ſame acid of nitre poured on ſnow inſtantly diſſolves it and produces the greateſt degree of cold yet known, by which at Peterſburgh quickſilver was firſt frozen in 1760.

Water may be cooled below 32° without being frozen, if it be placed on a ſolid floor and ſecured from agitation, but when thus cooled below the freezing point the leaſt [14] agitation turns part of it ſuddenly into ice, and when this ſudden freezing takes place a thermometer placed in it inſtantly riſes as ſome heat is given out in the act of congelation, and the ice is thus left with the ſame ſenſible degree of cold as the water had poſſeſſed before it was agitated, but is nevertheleſs now combined with leſs latent heat.

A cubic inch of water thus cooled down to 32° mixed with an equal quantity of boil⯑ing water at 212° will cool it to the middle number between theſe two, or to 122. But a cubit inch of ice whoſe ſenſible cold alſo is but 32, mixed with an equal quantity of boiling water, will cool it ſix times as much as the cubic inch of cold water above⯑mentioned, as the ice not only gains its ſhare of the ſenſible or gravitating heat of the boiling water but attracts to itſelf alſo and combines with the quantity of latent heat which it had loſt at the time of its congelation.

So boiling water will acquire but 212° of heat under the common preſſure of the at⯑moſphere, but the ſteam raiſed from it by its expanſion or by its ſolution in the atmo⯑ſphere combines with and carries away a prodigious quantity of heat which it again parts with on its condenſation; as is ſeen in common diſtillation where the large quantity of water in the worm-tub is ſo ſoon heated. Hence the evaporation of ether on a ther⯑mometer ſoon ſinks the mercury below freezing, and hence a warmth of the air in winter frequently ſucceeds a ſhower.

When the matter of heat or calorique is ſet at liberty from its combinations, as by inflammation, it paſſes into the ſurrounding bodies, which poſſeſs different capacities of acquiring their ſhare of the looſe or ſenſible heat; thus a pint meaſure of cold water at 48° mixed with a pint of boiling water at 212° will cool it to the degree between theſe two numbers, or to 154°, but it requires two pint meaſures of quickſilver at 48° of heat to cool one pint of water as above. Theſe and other curious experiments are adduced by Dr. Black to evince the exiſtance of combined or latent heat in bodies, as has been ex⯑plained by ſome of his pupils, and well illuſtrated by Dr. Crawford. The world has long been in expectation of an account of his diſcoveries on this ſubject by the celebrated author himſelf.

As this doctrine of elementary heat in its fluid and combined ſtate is not yet univer⯑ſally received, I ſhall here add two arguments in ſupport of it drawn from different ſources, viz. from the heat given out or abſorbed by the mechanical condenſation or expanſion of the air, and perhaps of other bodies, and from the analogy of the various phenomena of heat with thoſe of electricity.

I. If a thermometer be placed in the receiver of an air-pump, and the air haſtily ex⯑hauſted, the thermometer will ſink ſome degrees, and the glaſs become ſteamy; the ſame occurs in haſtily admitting a part of the air again. This I ſuppoſe to be produced by the expanſion of part of the air, both during the exhauſtion and re-admiſſion of it; and that the air ſo expanded becomes capable of attracting from the bodies in its vicinity a part of their heat, hence the vapours contained in it and the glaſs receiver are for a time colder and the ſteam is precipitated. That the air thus parts with its moiſture from the cold occaſioned by its rarefaction and not ſimply by the rarefaction itſelf is evident, becauſe in a minute or two the ſame rarefied air will again take up the dew depoſited on the receiver; and becauſe water will evaporate ſooner in rare than in denſe air.

[15] There is a curious phenomenon ſimilar to this obſerved in the fountain of Hiero con⯑ſtructed on a large ſcale at the Chemnicenſian mines in Hungary. In this machine the air in a large veſſel is compreſſed by a column of water 260 feet high, a ſtop-cock is then opened, and as the air iſſues out with great vehemence, and thus becomes immediately greatly expanded, ſo much cold is produced that the moiſture from this ſtream of air is precipitated in the form of ſnow, and ice is formed adhering to the noſel of the cock. This remarkable circumſtance is deſcribed at large with a plate of the machine in Philos. Trans. Vol. LII. for 1761.

The following experiment is related by Dr. Darwin in the Philos. Trans. Vol. LXXVIII. Having charged an air-gun as forcibly as he well could the air-cell and ſyringe became exceedingly hot, much more ſo than could be aſcribed to the friction in working it; it was then left about half an hour to cool down to the temperature of the air, and a thermometer having been previouſly fixed againſt a wall, the air was diſcharged in a con⯑tinual ſtream on its bulb, and it ſunk many degrees. From theſe three experiments of the ſteam in the exhauſtéd receiver being depoſited and re-abſorbed, when a part of the air is exhauſted or re-admitted, and the ſnow produced by the fountain of Hiero, and the extraordinary heat given out in charging, and the cold produced in diſcharging an air-gun, there is reaſon to conclude that when air is mechanically compreſſed the elementary fluid heat is preſſed out of it, and that when it is mechanically expanded the ſame fluid heat is re-abſorbed from the common maſs.

It is probable all other bodies as well as air attract heat from their neighbours when they are mechanically expanded, and give it out when they are mechanically condenſed. Thus when a vibration of the particles of hard bodies is excited by friction or by per⯑cuſſion, theſe particles mutually recede from and approach each other reciprocally; at the times of their receſſion from each other, the body becomes enlarged in bulk, and is then in a condition to attract heat from thoſe in its vicinity with great and ſudden power; at the times of their approach to each other this heat is again given out, but the bodies in contact having in the mean while received the heat they had thus loſt, from other bodies behind them, do not ſo ſuddenly or ſo forcibly re-abſorb the heat again from the body in vibration; hence it remains on its ſurface like the electric fluid on a rubbed glaſs globe, and for the ſame reaſon, becauſe there is no good conductor to take it up again. Hence at every vibration more and more heat is acquired and ſtands looſe upon the ſur⯑face; as in filing metals or rubbing glaſs tubes; and thus a ſmith with a few ſtrokes on a nail on his anvil can make it hot enough to light a brimſtone-match; and hence in ſtriking flint and ſteel together heat enough is produced to vitrify the parts thus ſtrucken off, the quantity of which heat is again probably increaſed by the new chemical com⯑bination.

II. The analogy between the phenomena of the electric fluid and of heat furniſhes another argument in ſupport of the exiſtence of heat as a gravitating fluid. 1. They are both accumulated by friction on the excited body. 2. They are propagated eaſily or with difficalty along the ſame claſſes of bodies; with eaſe by metals, with leſs eaſe by water; and with difficulty by reſins, bees-wax, ſilk, air, and glaſs. Thus glaſs canes or canes of ſealing-wax may be melted by a blow-pipe or a candle within a quarter of an [16] inch of the fingers which hold them, without any inconvenient heat, while a pin or other metallic ſubſtance applyed to the flame of a candle ſo readily conducts the heat as immediately to burn the fingers. Hence clothes of ſilk keep the body warmer than clothes of linen of equal thickneſs, by confining the heat upon the body. And hence plains are ſo much warmer than the ſummits of mountains by the greater denſity of the air con⯑fining the acquired heat upon them. 3. They both give out light in their paſſage through air, perhaps not in their paſſage through a vacuum. 4. They both of them fuſe or vitrify metals. 5. Bodies after being electrized if they are mechanically ex⯑tended will receive a greater quantity of electricity, as in Dr. Franklin's experiment of the chain in the tankard; the ſame ſeems true in reſpect to heat as explained above. 6. Both heat and electricity contribute to ſuſpend ſteam in the atmoſphere by producing or increaſing the repulſion of its particles. 7. They both gravitate, when they have been accumulated, till they find their equilibrium.

If we add to the above the many chemical experiments which receive an eaſy and ele⯑gant explanation from the ſuppoſed matter of heat, as employed in the works of Bergman and Lavoiſier, I think we may reaſonably allow of its exiſtence as an element, occaſionally combined with other bodies, and occaſionally exiſting as a fluid, like the electric fluid gravitating amongſt them, and that hence it may be propagated from the central fires of the earth to the whole maſs, and contribute to preſerve the mean heat of the earth, which in this country is about 48 degrees but variable from the greater or leſs effect of the ſun's heat in different climates, ſo well explained in Mr. Kirwan's Treatiſe on the Temperature of different Latitudes. 1787, Elmſly. London.

NOTE VIII.—MEMNON'S LYRE.

THE gigantic ſtatue of Memnon in his temple at Thebes had a lyre in his hands, which many credible writers aſſure us, ſounded when the riſing ſun ſhone upon it. Some philo⯑ſophers have ſuppoſed that the ſun's light poſſeſſes a mechanical impulſe, and that the ſounds abovementioned might be thence produced. Mr. Michell conſtructed a very tender hori⯑zontal balance, as related by Dr. Prieſtley in his hiſtory of light and colours, for this pur⯑poſe, but ſome experiments with this balance which I ſaw made by the late Dr. Powel, who threw the focus of a large reflector on one extremity of it, were not concluſive eitherway, as the copper leaf of the balance approached in one experiment and receded in another.

There are however methods by which either a rotative or alternating motion may be produced by very moderate degrees of heat. If a ſtraight glaſs tube, ſuch as are uſed for barometers, be ſuſpended horizontally before a fire, like a roaſting ſpit, it will revolve by intervals; for as glaſs is a bad conductor of heat the ſide next the fire becomes heated ſooner than the oppoſite ſide, and the tube becomes bent into a bow with the external part of the curve towards the fire, this curve then falls down and produces a fourth part of a revolution of the glaſs tube, which thus revolves with intermediate pauſes.

Another alternating motion I have ſeen produced by ſuſpending a glaſs tube about eight inches long with bulbs at each end on a centre like a ſcale beam. This curious machine is filled about one third part with pureſt ſpirit of wine, the other two thirds being a vacuum, and is called a pulſe-glaſs, if it be placed in a box before the fire, ſo that either bulb, as it riſes, may become ſhaded from the fire, and expoſed to it when it deſcends, an alternate libration of it is produced. For ſpirit of wine in vacuo emits ſteam by a very ſmall degree of heat, and this ſteam forces the ſpirit beneath it up into the upper bulb, which therefore deſcends. It is probable ſuch a machine on a larger ſcale might be of uſe to open the doors or windows of hot-houſes or mellon-frames, when the air within them ſhould become too much heated, or might be employed in more important me⯑chanical purpoſes.

On travelling through a hot ſummer's day in a chaiſe with a box covered with leather on the fore-axle-tree, I obſerved, as the ſun ſhone upon the black leather, the box began to open its lid, which at noon roſe above a foot, and could not without great force be preſſed down; and which gradually cloſed again as the ſun declined in the evening. This I ſuppoſe might with ſtill greater facility be applied to the purpoſe of opening melon⯑frames or the ſaſhes of hot-houſes.

The ſtatue of Memnon was overthrown and ſawed in two by Cambyſes to diſcover its internal ſtructure, and is ſaid ſtill to exiſt. See Savary's Letters on Egypt. The trun⯑cated ſtatue is ſaid for many centuries to have ſaluted the riſing ſun with chearful tones, and the ſetting ſun with melancholy ones.

NOTE IX.—LUMINOUS INSECTS.

THERE are eighteen ſpecies of Lampyris or glow-worm, according to Linneus, ſome of which are found in almoſt every part of the world. In many of the ſpecies the females have no wings, and are ſuppoſed to be diſcovered by the winged males by their ſhining in the night. They become much more lucid when they put themſelves in motion, which would ſeem to indicate that their light is owing to their reſpiration; in which proceſs it is probable phoſphoric acid is produced by the combination of vital air with ſome part of the blood, and that light is given out through their tranſparent bodies by this ſlow internal combuſtion.

There is a fire-fly of the beetle-kind deſcribed in the Dict. Raiſonné under the name of Acudia, which is ſaid to be two inches long, and inhabits the Weſt-Indies and South America; the natives uſe them inſtead of candles, putting from one to three of them under a glaſs. Madam Merian ſays, that at Surinam the light of this fly is ſo great, that ſhe ſaw ſufficiently well by one of them to paint and finiſh one of the figures of them in her work on inſects. The largeſt and oldeſt of them are ſaid to become four inches long, and to ſhine like a ſhooting ſtar as they fly, and are thence called Lantern⯑bearers. The uſe of this light to the inſect itſelf ſeems to be that it may not fly againſt objects in the night; by which contrivance theſe inſects are enabled to procure their ſuſtenance either by night or day, as their wants may require, or their numerous enemies permit them; whereas ſome of our beetles have eyes adapted only to the night, and if they happen to come abroad too ſoon in the evening are ſo dazzled that they fly againſt every thing in their way. See note on Phoſphorus, No. X.

In ſome ſeas, as particularly about the coaſt of Malabar, as a ſhip floats along, it ſeems during the night to be ſurrounded with fire, and to leave a long tract of light behind it. Whenever the ſea is gently agitated it ſeems converted into little ſtars, every drop as it breaks emits light, like bodies electrified in the dark. Mr. Bomare ſays, that when he was at the port of Cettes in Languedoc, and bathing with a companion in the ſea after a very hot day, they both appeared covered with fire after every immerſion, and that laying his wet hand on the arm of his companion, who had not then dipped himſelf, the exact mark of his hand and fingers was ſeen in characters of fire. As numerous micro⯑ſcopic inſects are found in this ſhining water, its light has been generally aſcribed to them, though it ſeems probable that fiſh-ſlime in hot countries may become in ſuch a ſtate of incipient putrefaction as to give light, eſpecially when by agitation it is more ex⯑poſed to the air; otherwiſe it is not eaſy to explain why agitation ſhould be neceſſary to produce this marine light. See note on Phoſphorus No. X.

NOTE X.—PHOSPHORUS.

KUNCKEL, a native of Hamburgh, was the firſt who diſcovered to the world the proceſs for producing phoſphorus; though Brandt and Boyle were likewiſe ſaid to have previouſly had the art of making it. It was obtained from ſal microcoſmicum by evaporation in the form of an acid, but has ſince been found in other animal ſubſtances, as in the aſhes of bones, and even in ſome vegetables, as in wheat flour. Keir's chemical Dict. This phoſphoric acid is like all other acids united with vital air, and requires to be treated with charcoal or phlogiſton to deprive it of this air, it then becomes a kind of animal ſulphur, but of ſo inflammable a nature, that on the acceſs of air it takes fire ſpontaneouſly, and as it burns becomes again united with vital air, and re-aſſumes its form of phoſphoric acid.

As animal reſpiration ſeems to be a kind of ſlow combuſtion, in which it is probable that pboſphoric acid is produced by the union of phoſphorus with the vital air, ſo it is alſo probable that phoſphoric acid is produced in the excretory or reſpiratory veſſels of luminous inſects, as the glow-worm and fire-fly, and ſome marine inſects. From the ſame principle I ſuppoſe the light from putrid fiſh, as from the heads of hadocks, and from putrid veal, and from rotten wood in a certain ſtate of their putrefaction, is pro⯑duced, and phoſphorus thus ſlowly combined with air is changed into phoſphoric acid. The light from the Bolognian ſtone, and from calcined ſhells, and from white paper, and linen after having been expoſed for a time to the ſun's light, ſeem to produce either the phoſphoric or ſome other kind of acid from the ſulphurous or phlogiſtic matter which they contain. See note on Beccari's ſhells. l. 180.

There is another proceſs ſeems ſimilar to this ſlow combuſtion, and that is bleaching. By the warmth and light of the ſun the water ſprinkled upon linen or cotton cloth ſeems to be decompoſed, (if we credit the theory of M. Lavoiſier,) and a part of the vital air thus ſet at liberty and uncombined and not being in its elaſtic form, more eaſily diſſolves the colouring or phlogiſtic matter of the cloth, and produces a new acid, which is itſelf colourleſs, or is waſhed out of the cloth by water. The new proceſs of bleaching con⯑firms a part of this theory, for by uniting much vital air to marine acid by diſtilling it from manganeſe, on dipping the cloth to be bleached in water repleat with this ſuper⯑aerated marine acid, the colouring matter diſappears immediately, ſooner indeed in cotton than in linen. See note XXXIV.

There is another proceſs which I ſuſpect bears analogy to theſe above-mentioned, and that is the rancidity of animal fat, as of bacon; if bacon be hung up in a warm kitchen, with much ſalt adhering on the outſide of it, the fat part of it ſoon becomes [20] yellow and rancid; if it be waſhed with much cold water after it has imbibed the ſalt, and juſt before it is hung up, I am well informed, that it will not become rancid, or in very ſlight degrees. In the former caſe I imagine the ſalt on the ſurface of the bacon attracts water during the cold of the night, which is evaporated during the day, and that in this evaporation a part of the water becomes decompoſed, as in bleaching, and its vital air uniting with greater facility in its unelaſtic ſtate with the animal fat, produces an acid, perhaps of the phoſphoric kind, which being of a fixed nature lies upon the bacon, giving it the yellow colour and rancid taſte. It is remarkable that the ſuper⯑aerated marine acid does not bleach living animal ſubſtances, at leaſt it did not whiten a part of my hand which I for ſome minutes expoſed to it.

NOTE XI.—STEAM-ENGINE.

THE expanſive force of ſteam was known in ſome degree to the antients, Hero of Alexandria deſcribes an application of it to produce a rotative motion by the re-action of ſteam iſſuing from a ſphere mounted upon an axis, through two ſmall tubes bent into tangents, and iſſuing from the oppoſite ſides of the equatorial diameter of the ſphere, the ſphere was ſupplied with ſteam by a pipe communicating with a pan of boiling water, and entering the ſphere at one of its poles.

A french writer about the year 1630 deſcribes a method of raiſing water to the upper part of a houſe by filling a chamber with ſteam, and ſuffering it to condenſe of itſelf, but it ſeems to have been mere theory, as his method was ſcarcely practicable as he deſcribes it. In 1655 the Marquis of Worceſter mentions a method of raiſing water by fire in his Century of Inventions, but he ſeems only to have availed himſelf of the expanſive force and not to have known the advantages ariſing from condenſing the ſteam by an injection of cold water. This latter and moſt important improvement ſeems to have been made by Capt. Savery ſometime prior to 1698, for in that year his patent for the uſe of that invention was confirmed by act of parliament. This gentleman appears to have been the firſt who reduced the machine to practice and exhibited it in an uſeful form. This method conſiſted only in expelling the air from a veſſel by ſteam and condenſing the ſteam by an injection of cold water, which making a vacuum, the preſſure of the atmo⯑ſphere forced the water to aſcend into the ſteam-veſſel through a pipe of 24 to 26 feet [21] high, and by the admiſſion of denſe ſteam from the boiler, forcing the water in the ſteam⯑veſſel to aſcend to the height deſired. This conſtruction was defective becauſe it required very ſtrong veſſels to reſiſt the force of the ſteam, and becauſe an enormous quantity of ſteam was condenſed by coming in contact with the cold water in the ſteam-veſſel.

About or ſoon after that time M. Papin attempted a ſteam-engine on ſimilar principles but rather more defective in its conſtruction.

The next improvement was made very ſoon afterwards by Meſſrs. Newcomen and Cawley of Dartmouth, it conſiſted in employing for the ſteam-veſſel a hollow cylinder, ſhut at bottom and open at top, furniſhed with a piſton ſliding eaſily up and down in it, and made tight by oakum or hemp, and covered with water. This piſton is ſuſpended by chains from one end of a beam, moveable upon an axis in the middle of its length, to the other end of this beam are ſuſpended the pump-rods.

The danger of burſting the veſſels was avoided in this machine, as however high the water was to be raiſed it was not neceſſary to increaſe the denſity of the ſteam but only to enlarge the diameter of the cylinder.

Another advantage was, that the cylinder not being made ſo cold as in Savary's method, much leſs ſteam was loſt in filling it after each condenſation.

The machine however ſtill remained imperfect, for the cold water thrown into the cylinder acquired heat from the ſteam it condenſed, and being in a veſſel exhauſted of air it produced ſteam itſelf, which in part reſiſted the action of the atmoſphere on the piſton; were this remedied by throwing in more cold water the deſtruction of ſteam in the next filling of the cylinder would be proportionally increaſed. It has therefore in practice been found adviſeable not to load theſe engines with columns of water weighing more than ſeven pounds for each ſquare inch of the area of the piſton. The bulk of water when converted into ſteam remained unknown until Mr. J. Watt, then of Glaſgow, in 1764, determined it to be about 1800 times more rare than water. It ſoon occurred to Mr. Watt that a perfect engine would be that in which no ſteam ſhould be condenſed in filling the cylinder, and in which the ſteam ſhould be ſo perfectly cooled as to produce nearly a perfect vacuum.

Mr. Watt having aſcertained the degree of heat in which water boiled in vacuo, and under progreſſive degrees of preſſure, and inſtructed by Dr. Black's diſcovery of latent heat, having calculated the quantity of cold water neceſſary to condenſe certain quantities of ſteam ſo far as to produce the exhauſtion required, he made a communication from the cylinder to a cold veſſel previouſly exhauſted of air and water, into which the ſteam ruſhed by its elaſticity, and became immediately condenſed. He then adapted a cover to the cylinder and admitted ſteam above the piſton to preſs it down inſtead of air, and inſtead of applying water he uſed oil or greaſe to fill the pores of the oakum and to lubri⯑cate the cylinder.

He next applied a pump to extract the injection water, the condenſed ſteam, and the air, from the condenſing veſſel, every ſtroke of the engine.

[22] To prevent the cooling of the cylinder by the contact of the external air, he ſur⯑rounded it with a caſe containing ſteam, which he again protected by a covering of matters which conduct heat ſlowly.

This conſtruction preſented an eaſy means of regulating the power of the engine, for the ſteam being the acting power, as the pipe which admits it from the boiler is more or leſs opened, a greater or ſmaller quantity can enter during the time of a ſtroke, and conſequently the engine can act with exactly the neceſſary degree of energy.

Mr. Watt gained a patent for his engine in 1768, but the further perſecution of his deſigns were delayed by other avocations till 1775, when in conjunction with Mr. Boulton of Soho near Birmingham, numerous experiments were made on a large ſcale by their united ingenuity, and great improvements added to the machinery, and an act of parlia⯑ment obtained for the prolongation of their patent for twenty-five years, they have ſince that time drained many of the deep mines in Cornwall, which but for the happy union of ſuch genius muſt immediately have ceaſed to work. One of theſe engines works a pump of eighteen inches diameter, and upwards of 100 fathom or 600 feet high, at the rate of ten to twelve ſtrokes of ſeven feet long each, in a minute, and that with one fifth part of the coals which a common engine would have taken to do the ſame work. The power of this engine may be eaſier comprehended by ſaying that it raiſed a weight equal to 81000 pounds 80 feet high in a minute, which is equal to the combined action of 200 good horſes. In Newcomen's engine this would have required a cylinder of the enormous diameter of 120 inches or ten feet, but as in this engine of Mr. Watt and Mr. Boulton the ſteam acts, and a vacuum is made, alternately above and below the piſton, the power exerted is double to what the ſame cylinder would otherways pro⯑duce, and is further augmented by an inequality in the length of the two ends of the lever.

Theſe gentlemen have alſo by other contrivances applied their engines to the turning of mills for almoſt every purpoſe, of which that great pile of machinery the Albion Mill is a well known inſtance. Forges, ſlitting mills, and other great works are erected where nature has furniſhed no running water, and future times may boaſt that this grand and uſeful engine was invented and perfected in our own country.

Since the above article went to the preſs the Albion Mill is no more; it is ſuppoſed to have been ſet on fire by intereſted or malicious incendaries, and is burnt to the ground. Whence London has loſt the credit and the advantage of poſſeſſing the moſt powerful machine in the world!

NOTE XII.—FROST.

THE cauſe of the expanſion of water during its converſion into ice is not yet well aſcertained, it was ſuppoſed to have been owing to the air being ſet at liberty in the act of congelation which was before diſſolved in the water, and the many air bubbles in ice were thought to countenance this opinion. But the great force with which ice expands during its congelation, ſo as to burſt iron bombs and coehorns, according to the experi⯑ments of Major Williams at Quebec, invalidates this idea of the cauſe of it, and may ſometime be brought into uſe as a means of breaking rocks in mining, or projecting cannon-balls, or for other mechanical purpoſes, if the means of producing congelation ſhould ever be diſcovered to be as eaſy as the means of producing combuſtion.

Mr. de Mairan attributes the increaſe of bulk of frozen water to the different arrange⯑ment of the particles of it in cryſtallization, as they are conſtantly joined at an angle of 60 degrees; and muſt by this diſpoſition he thinks occupy a greater volume than if they were parallel. He found the augmentation of the water during freezing to amount to one-fourteenth, one-eighteenth, one-nineteenth, and when the water was previouſly purged of air to only one-twenty-ſecond part. He adds that a piece of ice, which was at firſt only one-fourteenth part ſpecifically lighter than water, on being expoſed ſome days to the froſt became one-twelfth lighter than water. Hence he thinks ice by being expoſed to greater cold ſtill increaſes in volume, and to this attributes the burſting of ice in ponds and on the glaciers. See Lewis's Commerce of Arts, p. 257. and the note on Muſchus in the other volume of this work.

This expanſion of ice well accounts for the greater miſchief done by vernal froſts at⯑tended with moiſture, (as by hoar-froſts,) than by the dry froſts called black froſts. Mr. Lawrence in a letter to Mr. Bradley complains that the dale-miſt attended with a froſt on may-day had deſtroyed all his tender fruits; though there was a ſharper froſt the night before without a miſt, that did him no injury; and adds, that a garden not a ſtone's throw from his own on a higher ſituation, being above the dale-miſt, had received no damage. Bradley, Vol. II. p. 232.

Mr. Hunter by very curious experiments diſcovered that the living principle in fiſh, in vegetables, and even in eggs and ſeeds, poſſeſſes a power of reſiſting congelation. Phil. Tranſ. There can be no doubt but that the exertions of animals to avoid the pain of cold may produce in them a greater quantity of heat, at leaſt for a time, but that vegetables, eggs, or ſeeds, ſhould poſſeſs ſuch a quality is truly wonderful. Others have imagined that animals poſſeſs a power of preventing themſelves from becom⯑ing much warmer than 98 degrees of heat, when immerſed in an atmoſphere above that degree of heat. It is true that the increaſed exhalation from their bodies will in ſome meaſure cool them, as much heat is carried off by the evaporation of fluids, but this is a chemical not an animal proceſs. The experiments made by thoſe who continued [24] many minutes in the air of a room heated ſo much above any natural atmoſpheric heat, do not ſeem concluſive, as they remained in it a leſs time than would have been neces⯑ſary to have heated a maſs of beef of the ſame magnitude, and the circulation of the blood in living animals, by perpetually bringing new ſupplies of fluid to the ſkin, would prevent the external ſurface from becoming hot much ſooner than the whole maſs. And thirdly, there appears no power of animal bodies to produce cold in diſeaſes, as in ſcarlet fever, in which the increaſed action of the veſſels of the ſkin produces heat and contributes to ex⯑hauſt the animal power already too much weakened.

It has been thought by many that froſts meliorate the ground, and that they are in general ſalubrious to mankind. In reſpect to the former it is now well known that ice or ſnow contain no nitrous particles, and though froſt by enlarging the bulk of moiſt clay leaves it ſofter for a time after the thaw, yet as ſoon as the water exhales, the clay becomes as hard as before, being preſſed together by the incumbent atmoſphere, and by its ſelf⯑attraction, called ſetting by the potters. Add to this that on the coaſts of Africa, where froſt is unknown, the fertility of the ſoil is almoſt beyond our conceptions of it. In reſpect to the general ſalubrity of froſty ſeaſons the bills of mortality are an evidence in the negative, as in long froſts many weakly and old people periſh from debility occaſioned by the cold, and many claſſes of birds and other wild animals are benumbed by the cold or deſtroyed by the conſequent ſcarcity of food, and many tender vegetables periſh from the degree of cold.

I do not think it ſhould be objected to this doctrine that there are moiſt days attended with a briſk cold wind when no viſible ice appears, and which are yet more diſagreeable and deſtructive than froſty weather. For on theſe days the cold moiſture, which is de⯑poſited on the ſkin is there evaporated and thus produces a degree of cold perhaps greater than the milder froſts. Whence even in ſuch days both the diſagreeable ſenſations and inſalubrious effects belong to the cauſe abovementioned, viz. the intenſity of the cold. Add to this that in theſe cold moiſt days as we paſs along or as the wind blows upon us, a new ſheet of cold water is as it were perpetually applied to us and hangs upon our bodies, now as water is 800 times denſer than air and is a much better conductor of heat, we are ſtarved with cold like thoſe who go into a cold bath, both by the great number of particles in contact with the ſkin and their greater facility of receiving our heat.

It may nevertheleſs be true that ſnows of long duration in our winters may be leſs in⯑jurious to vegetation than great rains and ſhorter froſts, for two reaſons. 1. Becauſe great rains carry down many thouſand pounds worth of the beſt part of the manure off the lands into the ſea, whereas ſnow diſſolves more gradually and thence carries away leſs from the land; any one may diſtinguiſh a ſnow-flood from a rain-flood by the tranſparency of the water. Hence hills or fields with conſiderable inclination of ſurface ſhould be ploughed horizontally that the furrows may ſtay the water from ſhowers till it depoſits its mud. 2. Snow protects vegetables from the ſeverity of the froſt, ſince it is always in a ſtate of thaw where it is in contact with the earth; as the earth's heat is about 48° and the heat of thawing ſnow is 32° the vegetables between them are kept in a degree of heat about 40, by which many of them are preſerved. See note on Muſchus, Vol. II. of this work.

NOTE XIII.—ELECTRICITY.

NOTE XIV.—BUDS AND BULBS.

A TREE is properly ſpeaking a family or ſwarm of buds, each bud being an in⯑dividual plant, for if one of theſe buds be torn or cut out and planted in the earth with a glaſs cup inverted over it to prevent its exhalation from being at firſt greater than its power of abſorption, it will produce a tree ſimilar to its parent; each bud has a leaf, which is its lungs, appropriated to it, and the bark of the tree is a congeries of the roots of theſe individual buds, whence old hollow trees are often ſeen to have ſome branches flouriſh with vigour after the internal wood is almoſt intirely decayed and vaniſhed. According to this idea Linneus has obſerved that trees and ſhrubs are roots above ground, for if a tree be inverted leaves will grow from the root-part and roots from the trunk-part. Phil. Bot. p. 39. Hence it appears that vegetables have two methods of pro⯑pagating themſelves, the oviparous as by ſeeds, and the viviparous as by their buds and bulbs, and that the individual plants, whether from ſeeds or buds or bulbs, are all annual productions like many kinds of inſects as the ſilk-worm, the parent periſhing in the autumn after having produced an embryon, which lies in a torpid ſtate during the winter, and is matured in the ſucceeding ſummer. Hence Linneus names buds and bulbs the winter-cradles of the plant or hybernacula, and might have given the ſame term to ſeeds. In warm climates few plants produce buds, as the vegetable life can be com⯑pleated in one ſummer, and hence the hybernacle is not wanted; in cold climates alſo ſome plants do not produce buds, as philadelphus, ſrangula, viburnum, ivy, heath, wood-nightſhade, rue, geranium.

The bulbs of plants are another kind of winter-cradle, or hybernacle, adhering to the deſcending trunk, and are found in the perennial herbaceous plants which are too tender to bear the cold of the winter. The production of theſe ſubterraneous winter lodges, is not yet perhaps clearly underſtood, they have been diſtributed by Linneus according to their forms into ſcaly, ſolid, coated, and jointed bulbs, which however does not elucidate their manner of production. As the buds of trees may be truly eſteemed individual annual plants, their roots conſtituting the bark of the tree, it follows that theſe roots (viz. of each individual bud) ſpread themſelves over the laſt years bark, making a new bark over the old one, and thence deſcending cover with a new bark the old roots alſo in the ſame manner. A ſimilar circumſtance I ſuppoſe to happen in ſome herbaceous plants, that is, a new bark is annually produced over the old root, and thus for ſome years at leaſt the old root or caudex increaſes in ſize and puts up new ſtems. As theſe roots increaſe in ſize the central part I ſuppoſe changes like the in⯑ternal wood of a tree and does not poſſeſs any vegetable life, and therefore gives out no fibres or rootlets, and hence appears bitten off, as in valerian, plantain, and devil's-bit. And this decay of the central part of the root I ſuppoſe has given occaſion to the belief of the root-fibres drawing down the bulb ſo much inſiſted on by Mr. Milne in his Botanical Dictionary, Art. Bulb.

[28] From the obſervations and drawings of various kinds of bulbous roots at different times of their growth, ſent me by a young lady of nice obſervation, it appears probable that all bulbous roots properly ſo called periſh annually in this climate: Bradley, Miller, and the Author of Spectacle de la Nature, obſerve that the tulip annually renews its bulb, for the ſtalk of the old flower is found under the old dry coat but on the outſide of the new bulb. This large new bulb is the flowering bulb, but beſides this there are other ſmall new bulbs produced between the coats of this large one but from the ſame caudex, (or circle from which the root-fibres ſpring;) theſe ſmall bulbs are leaf-bearing bulbs, and renew themſelves annually with increaſing ſize till they bear flowers.

Miſs—favoured me with the following curious experiment: She took a ſmall tulip-root out of the earth when the green leaves were ſufficiently high to ſhow the flower, and placed it in a glaſs of water; the leaves and flower ſoon withered and the bulb became wrinkled and ſoft, but put out one ſmall ſide bulb and three bulbs beneath deſcending an inch into the water by long proceſſes from the caudex, the old bulb in ſome weeks intirely decayed; on diſſecting this monſter, the middle deſcending bulb was found by its proceſs to adhere to the caudex and to the old flower-ſtem, and the ſide ones were ſeparated from the flower-ſtem by a few ſhrivelled coats but adhered to the caudex. Whence ſhe concludes that theſe laſt were off-ſets or leaf-bulbs which ſhould have been ſeen between the coats of the new flower-bulb if it had been left to grow in the earth, and that the middle one would have been the new flower-bulb. In ſome years (perhaps in wet ſeaſons) the floriſts are ſaid to loſe many of their tulip-roots by a ſimilar proceſs, the new leaf-bulbs being produced beneath the old ones by an elongation of the caudex without any new flower-bulbs.

By repeated diſſections ſhe obſerves that the leaf-bulbs or off-ſets of tulip, crocus, gladiolus, fritillary, are renewed in the ſame manner as the flowering-bulbs, contrary to the opinion of many writers; this new leaf-bulb is formed on the inſide of the coats from whence the leaves grow, and is more or leſs advanced in ſize as the outer coats and leaves are more or leſs ſhrivelled. In examining tulip, iris, hyacinth, hare-bell, the new bulb was invariably found between the flower-ſtem and the baſe of the innermoſt leaf of thoſe roots which had flowered, and incloſed by the baſe of the innermoſt leaf in thoſe roots which had not flowered, in both caſes adhering to the caudex or fleſhy circle from which the root-fibres ſpring.

Hence it is probable that the bulbs of hyacinths are renewed annually, but that this is performed from the caudex within the old bulb, the outer coat of which does not ſo ſhrivel as in crocus and fritillary and hence this change is not ſo apparent. But I believe as ſoon as the flower is advanced the new bulbs may be ſeen on diſſection, nor does the annual increaſe of the ſize of the root of cyclamen and of aletris capenſis mili⯑tate againſt this annual renewal of them, ſince the leaf-bulbs or off-ſets, as deſcribed above, are increaſed in ſize as they are annually renewed. See note on orchis, and on anthoxanthum, in Vol. II. of this work.

NOTE XV.—SOLAR VOLCANOS.

DR. ALEXANDER WILSON, Profeſſor of Aſtronomy at Glaſgow, publiſhed a paper in the Philoſophical Tranſactions for 1774, demonſtrating that the ſpots in the ſun's diſk are real cavities, excavations through the luminous material, which covers the other parts of the ſun's ſurface. One of theſe cavities he found to be about 4000 miles deep and many times as wide. Some objections were made to this doctrine by M. De la Laude in the Memoirs of the French Academy for the year 1776, which however have been ably anſwered by Profeſſor Wilſon in reply in the Philoſ. Tranſ, for 1783. Keil obſerves, in his Aſtronomical Lectures, p. 44, ‘We frequently ſee ſpots in the ſun which are larger and broader not only than Europe or Africa, but which even equal, if they do not exceed, the ſurface of the whole terraqueous globe.’ Now that theſe cavities are made in the ſun's body by a proceſs of nature ſimilar to our earthquakes does not ſeem improbable on ſeveral accounts. 1. Becauſe from this diſcovery of Dr. Wilſon it appears that the internal parts of the ſun are not in a ſtate of inflammation or of ejecting light, like the external part or luminous ocean which covers it; and hence that a greater degree of heat or inflammation and conſequent expanſion or exploſion may occaſionally be produced in its internal or dark nucleus. 2. Becauſe the ſolar ſpots or cavities are frequently increaſed or diminiſhed in ſize. 3. New ones are often pro⯑duced. 4. And old ones vaniſh. 5. Becauſe there are brighter or more luminous parts of the ſun's diſk, called faculae by Scheiner and Hevelius, which would ſeem to be vol⯑canos in the ſun, or, as Dr. Wilſon calls them, ‘eructations of matter more luminous than that which covers the ſun's ſurface.’ 6. To which may be added that all the planets added together with their ſatellites do not amount to more than one ſix hundred and fiftieth part of the maſs of the ſun according to Sir Iſaac Newton.

Now if it could be ſuppoſed that the planets were originally thrown out of the ſun by larger ſun-quakes than thoſe frequent ones which occaſion theſe ſpots or excavations above-mentioned, what would happen? I. According to the obſervations and opinion of Mr. Herſchel the ſun itſelf and all its planets are moving forwards round ſome other centre with an unknown velocity, which may be of opake matter correſponding with the very antient and general idea of a chaos. Whence if a ponderous planet, as Saturn, could be ſuppoſed to be projected from the ſun by an exploſion, the motion of the ſun itſelf might be at the ſame time diſturbed in ſuch a manner as to prevent the planet from falling again into it. 2. As the ſun revolves round its own axis its form muſt be that of an oblate ſpheroid like the earth, and therefore a body projected from its ſurface perpendicularly upwards from that ſurface would not riſe perpendicularly from the ſun's centre, unleſs it happened to be projected exactly from either of its poles or [30] from its equator. Whence it may not be neceſſary that a planet if thus projected from the ſun by exploſion ſhould again fall into the ſun. 3. They would part from the ſun's ſurface with the velocity with which that ſurface was moving, and with the velocity acquired by the exploſion, and would therefore move round the ſun in the ſame direction in which the ſun rotates on its axis, and perform eliptic orbits. 4. All the planets would move the ſame way round the ſun, from this firſt motion acquired at leaving its ſurface, but their orbits would be inclined to each other according to the diſtance of the part, where they were thrown out, from the ſun's equator. Hence thoſe which were ejected near the ſun's equator would have orbits but little inclined to each other, as the primary planets; the plain of all whoſe orbits are inclined but ſeven degrees and a half from each other. Others which were ejected near the ſun's poles would have much more eccentric orbits, as they would partake ſo much leſs of the ſun's rotatory motion at the time they parted from his ſurface, and would therefore be carried further from the ſun by the velocity they had gained by the exploſion which ejected them, and become comets. 5. They would all obey the ſame laws of motion in their revolutions round the ſun; this has been determined by aſtronomers, who have demonſtrated that they move through equal areas in equal times. 6. As their annual periods would depend on the height they roſe by the exploſion, theſe would differ in them all. 7. As their diurnal revolutions would depend on one ſide of the exploded matter adhering more than the other at the time it was torn off by the exploſion, theſe would alſo differ in the different planets, and not bear any proportion to their annual periods. Now as all theſe circumſtances coincide with the known laws of the planetary ſyſtem, they ſerve to ſtrengthen this conjecture.

This coincidence of ſuch a variety of circumſtances induced M. de Buffon to ſuppoſe that the planets were all ſtruck off from the ſun's ſurface by the impact of a large comet, ſuch as approached ſo near the ſun's diſk, and with ſuch amazing velocity, in the year 1680, and is expected to return in 2255. But Mr. Buffon did not recollect that theſe comets themſelves are only planets with more eccentric orbits, and that therefore it muſt be aſked, what had previouſly ſtruck off theſe comets from the ſun's body? 2. That if all theſe planets were ſtruck off from the ſun at the ſame time, they muſt have been ſo near as to have attracted each other and have formed one maſs: 3. That we ſhall want new cauſes for ſeparating the ſecondary planets from the primary ones, and muſt there⯑fore look out for ſome other agent, as it does not appear how the impulſe of a comet could have made one planet roll round another at the time they both of them were driven off from the ſurface of the ſun.

If it ſhould be aſked, why new planets are not frequently ejected from the ſun? it may be anſwered, that after many large earthquakes many vents are left for the elaſtic vapours to eſcape, and hence, by the preſent appearance of the ſurface of our earth, earthquakes prodigiouſly larger than any recorded in hiſtory have exiſted; the ſame cir⯑cumſtances may have affected the ſun, on whoſe ſurface there are appearances of vol⯑canos, as deſcribed above. Add to this, that ſome of the comets, and even the georgium ſidus, may, for ought we know to the contrary, have been emitted from the ſun in more [31] modern days, and have been diverted from their courſe, and thus prevented from re⯑turning into the ſun, by their approach to ſome of the older planets, which is ſomewhat countenanced by the opinion ſeveral philoſophers have maintained, that the quantity of matter of the ſun has decreaſed. Dr. Halley obſerved, that by comparing the proportion which the periodical time of the moon bore to that of the ſun in former times, with the proportion between them at preſent, that the moon is found to be ſomewhat accelerated in reſpect to the ſun. Pemberton's View of Sir Iſaac Newton, p. 247. And ſo large is the body of this mighty luminary, that all the planets thus thrown out of it would make ſcarcely any perceptible diminution of it, as mentioned above. The cavity mentioned above, as meaſured by Dr. Wilſon of 4000 miles in depth, not penetrating an hundredth part of the ſun's ſemi-diameter; and yet, as its width was many times greater than its depth, was large enough to contain a greater body than our terreſtrial world.

I do not mean to conceal, that from the laws of gravity unfolded by Sir Iſaac Newton, ſuppoſing the ſun to be a ſphere and to have no progreſſive motion, and not liable itſelf to be diſturbed by the ſuppoſed projection of the planets from it, that ſuch planets muſt return into the ſun. The late Rev. William Ludlam, of Leiceſter, whoſe genius never met with reward equal to its merits, in a letter to me, dated January, 1787, after having ſhewn, as mentioned above, that planets ſo projected from the ſun would return to it, adds, ‘That a body as large as the moon ſo projected, would diſturb the motion of the earth in its orbit, is certain; but the calculation of ſuch diſturbing forces is difficult. The body in ſome circumſtances might become a ſatellite, and both move round their common centre of gravity, and that centre be carried in an annual orbit round the ſun.’

There are other circumſtances which might have concurred at the time of ſuch ſup⯑poſed exploſions, which would render this idea not impoſſible. I. The planets might be thrown out of the ſun at the time the ſun itſelf was riſing from chaos, and be attracted by other ſuns in their vicinity riſing at the ſame time out of chaos, which would prevent them from returning into the ſun. 2. The new planet in its courſe or aſcent from the ſun, might explode and eject a ſatellite, or perhaps more than one, and thus by its courſe being affected might not return into the ſun. 3. If more planets were ejected at the ſame time from the ſun, they might attract and diſturb each others courſe at the time they left the body of the ſun, or very ſoon afterwards, when they would be ſo much nearer each other.

NOTE XVI.—CALCAREOUS EARTH.

FROM having obſerved that many of the higheſt mountains of the world conſiſt of lime-ſtone replete with ſhells, and that theſe mountains bear the marks of having been lifted up by ſubterraneous fires from the interior parts of the globe; and as lime-ſtone replete with ſhells is found at the bottom of many of our deepeſt mines ſome philo⯑ſophers have concluded that the nucleus of the earth was for many ages covered with water which was peopled with its adapted animals; that the ſhells and bones of theſe animals in a long ſeries of time produced ſolid ſtrata in the ocean ſurrounding the ori⯑ginal nucleus.

Theſe ſtrata conſiſt of the accumulated exuviae of ſhell-fiſh, the animals periſhed age after age but their ſhells remained, and in progreſſion of time produced the amazing quantities of lime-ſtone which almoſt cover the earth. Other marine animals called coralloids raiſed walls and even mountains by the congeries of their calcareous habita⯑tions, theſe perpendicular corralline rocks make ſome parts of the Southern Ocean highly dangerous, as appears in the journals of Capt. Cook. From contemplating the immenſe ſtrata of lime-ſtone, both in reſpect to their extent and thickneſs, formed from theſe ſhells of animals, philoſophers have been led to conclude that much of the water of the ſea has been converted into calcareous earth by paſſing through their organs of digeſtion. The formation of calcareous earth ſeems more particularly to be an animal proceſs as the formation of clay belongs to the vegetable economy; thus the ſhells of crabs and other teſtaceous fiſh are annually reproduced from the mucous membrane beneath them; the ſhells of eggs are firſt a mucous membrane, and the calculi of the kidneys and thoſe found in all other parts of our ſyſtem which ſometimes contain calcareous earth, ſeem to originate from inflamed membranes; the bones themſelves conſiſt of calcareous earth united with the phoſphoric or animal acid, which may be ſeparated by diſſolving the aſhes of calcined bones in the nitrous acid; the various ſecretions of animals, as their ſaliva and urine, abound likewiſe with calcareous earth, as appears by the incruſtations about the teeth and the ſediments of urine. It is probable that animal mucus is a pre⯑vious proceſs towards the formation of calcareous earth; and that all the calcareous earth in the world which is ſeen in lime-ſtones, marbles, ſpars, alabaſters, marls, (which make up the greateſt part of the earth's cruſt, as far as it has yet been penetrated,) have been formed originally by animal and vegetable bodies from the maſs of water, and that by theſe means the ſolid part of the terraqueous globe has perpetually been in an increaſing ſtate and the water perpetually in a decreaſing one.

After the mountains of ſhells and other recrements of aquatic animals were elevated above the water the upper heaps of them were gradually diſſolved by rains and dews and oozing through were either perfectly cryſtallized in ſmaller cavities and formed [33] calcareous ſpar, or were imperfectly cryſtallized on the roofs of larger cavities and pro⯑duced ſtalactites; or mixing with other undiſſolved ſhells beneath them formed marbles, which were more or leſs cryſtallized and more or leſs pure; or laſtly, after being diſſolved, the water was exhaled from them in ſuch a manner that the external parts became ſolid, and forming an arch prevented the internal parts from approaching each other ſo near as to become ſolid, and thus chalk was produced. I have ſpecimens of chalk formed at the root of ſeveral ſtalactites, and in their central parts; and of other ſtalactites which are hollow like quills from a ſimilar cauſe, viz. from the external part of the ſtalactite harden⯑ing firſt by its evaporation, and thus either attracting the internal diſſolved particles to the cruſt, or preventing them from approaching each other ſo as to form a ſolid body. Of theſe I ſaw many hanging from the arched roof of a cellar under the high ſtreet in Edinburgh.

If this diſſolved limeſtone met with vitriolic acid it was converted into alabaſter, parting at the ſame time with its fixable air. If it met with the fluor acid it became fluor; if with the ſiliceous acid, flint; and when mixed with clay and ſand, or either of them, acquires the name of marl. And under one or other of theſe forms compoſes a great part of the ſolid globe of the earth.

Another mode in which limeſtone appears is in the form of round granulated par⯑ticles, but ſlightly cohering together; of this kind a bed extends over Lincoln heath, perhaps twenty miles long by ten wide. The form of this calcareous ſand, its angles having been rubbed off, and the flatneſs of its bed, evinces that that part of the country was ſo formed under water, the particles of ſand having thus been rounded, like all other rounded pebbles. This round form of calcareous ſand and of other larger pebbles is produced under water, partly by their being more or leſs ſoluble in water, and hence the angular parts become diſſolved, firſt, by their expoſing a larger ſurface to the action of the menſtruum, and ſecondly, from their attrition againſt each other by the ſtreams or tides, for a great length of time, ſucceſſively as they were collected, and perhaps when ſome of them had not acquired their hardeſt ſtate.

This calcareous ſand has generally been called ketton-ſtone and believed to reſemble the ſpawn of fiſh, it has acquired a form ſo much rounder than ſiliceous ſand from its being of ſo much ſofter a texture and alſo much more ſoluble in water. There are other ſoft calcareous ſtones called tupha which are depoſited from water on moſſes, as at Matlock, from which moſs it is probable the water may receive ſomething which induces it the readier to part with its earth.

In ſome lime-ſtones the living animals ſeem to have been buried as well as their ſhells during ſome great convulſion of nature, theſe ſhells contain a black coaly ſubſtance within them, in others ſome phlogiſton or volatile alcali from the bodies of the dead animals remains mixed with the ſtone, which is then called liver-ſtone as it emits a ſulphurous ſmell on being ſtruck, and there is a ſtratum about ſix inches thick extends a conſiderable way over the iron ore at Wingerworth near Cheſterfield in Derbyſhire which ſeems evidently to have been formed from the ſhells of freſh-water muſcles

[34] There is however another ſource of calcareous earth beſides the aquatic one above deſcribed and that is from the recrements of land animals and vegetables as found in marls, which conſiſt of various mixtures of calcareous earth, ſand, and clay, all of them perhaps principally from vegetable origin.

Dr. Hutton is of opinion that the rocks of marble have been ſoftened by fire into a fluid maſs, which he thinks under immenſe preſſure might be done without the eſcape of their carbonic acid or fixed air. Edinb. Tranſact. Vol. I. If this ingenious idea be allowed it might account for the purity of ſome white marbles, as during their fluid ſtate there might be time for their partial impurities, whether from the bodies of the animals which produced the ſhells or from other extraneous matter, either to ſublime to the uppermoſt part of the ſtratum or to ſubſide to the lowermoſt part of it. As a confirmation of this theory of Dr. Hutton's it may be added that ſome calcareous ſtones are found mixed with lime, and have thence loſt a part of their fixed air or carbonic gas, as the bath-ſtone, and on that account hardens on being expoſed to the air, and mixed with ſulphur produces calcareous liver of ſulphur. Falconer on Bath⯑water. Vol. I. p. 156. and p. 257. Mr. Monnet found lime in powder in the mountains of Auvergne, and ſuſpected it of volcanic origin. Kirwan's Min. p. 22.

NOTE XVII.—MORASSES.

WHERE woods have repeatedly grown and periſhed moraſſes are in proceſs of time produced, and by their long roots fill up the interſtices till the whole becomes for many yards deep a maſs of vegetation. This fact is curiouſly verified by an account given many years ago by the Earl of Cromartie, of which the following is a ſhort abſtract.

In the year 1651 the EARL OF CROMARTIE being then nineteen years of age ſaw a plain in the pariſh of Lockburn covered over with a firm ſtanding wood, which was ſo old that not only the trees had no green leaves upon them but the bark was totally thrown off, which he was there informed by the old countrymen was the univerſal manner in which fir-woods terminated, and that in twenty or thirty years the trees would caſt themſelves up by the roots. About fifteen years after he had occaſion to travel the ſame way and obſerved that there was not a tree nor the appearance of a root of any of them; but in their place the whole plain where the wood ſtood was [35] covered with a flat green moſs or moraſs, and on aſking the country people what was become of the wood he was informed that no one had been at the trouble to carry it away, but that it had all been overturned by the wind, that the trees lay thick over each other, and that the moſs or bog had overgrown the whole timber, which they added was occaſioned by the moiſture which came down from the high hills above it and ſtagnated upon the plain, and that nobody could yet paſs over it, which however his Lordſhip was ſo incautious as to attempt and ſlipt up to the arm-pits. Before the year 1699 that whole piece of ground was become a ſolid moſs wherein the peaſants then dug turf or peat, which however was not yet of the beſt ſort. Philos. Trans. No. 330. Abridg. Vol. V. p. 272.

Moraſſes in great length of time undergo variety of changes, firſt by elutriation, and afterwards by fermentation, and the conſequent heat. 1. By water perpetually oozing through them the moſt ſoluble parts are firſt waſhed away, as the eſſential ſalts, theſe together with the ſalts from animal recrements are carried down the rivers into the ſea, where all of them ſeem to decompoſe each other except the marine ſalt. Hence the aſhes of peat contain little or no vegetable alcali and are not uſed in the countries, where peat conſtitutes the fuel of the lower people, for the purpoſe of waſhing linen. The ſecond thing which is always ſeen oozing from moraſſes is iron in ſolution, which produces chalybeate ſprings, from whence depoſitions of ochre and variety of iron ores. The third elutriation ſeems to conſiſt of vegetable acid, which by means unknown appears to be converted into all other acids. 1. Into marine and nitrous acids as mentioned above. 2. Into vitriolic acid which is found in ſome moraſſes ſo plentifully as to preſerve the bodies of animals from putrefaction which have been buried in them, and this acid carried away by rain and dews and meeting with calcareous earth produces gypſum or alabaſter, with clay it produces alum, and deprived of its vital air produces ſulphur. 3. Fluor acid which being waſhed away and meeting with calcareous earth produces fluor or cubic ſpar. 4. The ſiliceous acid which ſeems to have been dis⯑ſeminated in great quantity either by ſolution in water or by ſolution in air, and appears to have produced the ſand in the ſea uniting with calcareous earth previouſly diſſolved in that element, from which were afterwards formed ſome of the grit-ſtone rocks by means of a ſiliceous or calcareous cement. By its union with the calcareous earth of the moraſs other ſtrata of ſiliceous ſand have been produced; and by the mixture of this with clay and lime aroſe the beds of marl.

In other circumſtances, probably where leſs moiſture has prevailed, moraſſes ſeem to have undergone a fermentation, as other vegetable matter, new hay for inſtance is liable to do from the great quantity of ſugar it contains. From the great heat thus produced in the lower parts of immenſe beds of moraſs the phlogiſtic part, or oil, or aſphaltum, becomes diſtilled, and riſing into higher ſtrata becomes again condenſed forming coal⯑beds of greater or leſs purity according to their greater or leſs quantity of inflammable matter; at the ſame time the clay beds become purer or leſs ſo, as the phlogiſtic part is more or leſs completely exhaled from them. Though coal and clay are frequently pro⯑duced in this manner, yet I have no doubt, but that they are likewiſe often produced by [36] elutriation; in ſituations on declivities the clay is waſhed away down into the valleys, and the phlogiſtic part or coal left behind; this circumſtance is ſeen in many valleys near the beds of rivers, which are covered recently by a whitiſh impure clay, called water⯑clay. See note XIX. XX. and XXIII.

LORD CROMARTIE has furniſhed another curious obſervation on moraſſes in the paper above referred to. In a moſs near the town of Eglin in Murray, though there is no river or water which communicates with the moſs, yet for three or four feet of depth in the moſs there are little ſhell-fiſh reſembling oyſters with living fiſh in them in great quantities, though no ſuch fiſh are found in the adjacent rivers, nor even in the water pits in the moſs, but only in the ſolid ſubſtance of the moſs. This curious fact not only accounts for the ſhells ſometimes found on the ſurface of coals, and in the clay above them; but alſo for a thin ſtratum of ſhells which ſometimes exiſts over iron-ore.

NOTE XVIII.—IRON.

AS iron is formed near the ſurface of the earth, it becomes expoſed to ſtreams of water and of air more than moſt other metallic bodies, and thence becomes combined with oxygene, or vital air, and appears very frequently in its calciform ſtate, as in variety of ochres. Manganeſe, and zinc, and ſometimes lead, are alſo found near the ſurface of the earth, and on that account become combined with vital air and are exhibited in their calciform ſtate.

The avidity with which iron unites with oxygene, or vital air, in which proceſs much heat is given out from the combining materials, is ſhewn by a curious experiment of M. Ingenhouz. A fine iron wire twiſted ſpirally is fixed to a cork, on the point of the ſpire is fixed a match made of agaric dipped in ſolution of nitre; the match is then ignited, and the wire with the cork put immediately into a bottle full of vital air, the match firſt burns vividly, and the iron ſoon takes fire and conſumes with brilliant ſparks till it is reduced to ſmall brittle globules, gaining an addition of about one third of its weight by its union with vital air. Annales de Chymic. Traité de Chimie, per Lavoiſier, c. iii.

NOTE XIX.—FLINT.

NOTE XX.—CLAY.

THE philoſophers, who have attended to the formation of the earth, have ac⯑knowledged two great agents in producing the various changes which the terraqueous globe has undergone, and theſe are water and fire. Some of them have perhaps aſcribed too much to one of theſe great agents of nature, and ſome to the other. They have generally agreed that the ſtratification of materials could only be produced from ſediments or precipitations, which were previouſly mixed or diſſolved in the ſea; and that whatever effects were produced by fire were performed afterwards.

There is however great difficulty in accounting for the univerſal ſtratification of the ſolid globe of the earth in this manner, ſince many of the materials, which appear in ſtrata, could not have been ſuſpended in water; as the nodules of flint in chalk-beds, the extenſive beds of ſhells, and laſtly the ſtrata of coal, clay, ſand, and iron-ore, which in moſt coal-countries lie from five to ſeven times alternately ſtratified over each other, and none of them are ſoluble in water. Add to this if a ſolution of them or a mixture of them in water could be ſuppoſed, the cauſe of that ſolution muſt ceaſe before a precipitation could commence.

1. The great maſſes of lava, under the various names of granite, porphyry, toadſtone, moor-ſtone, rag, and ſlate, which conſtitute the old word, may have acquired the ſtrati⯑fication, which ſome of them appear to poſſeſs, by their having been formed by ſuc⯑ceſſive eruptions of a fluid maſs, which at different periods of antient time aroſe from volcanic ſhafts and covered each other, the ſurface of the interior maſs of lava would cool and become ſolid before the ſuperincumbent ſtratum was poured over it; to the ſame cauſe may be aſcribed their different compoſitions and textures, which are ſcarcely the ſame in any two parts of the world.

2. The ſtratifications of the great maſſes of limeſtone, which were produced from ſea-ſhells, ſeem to have been formed by the different times at which the innumerable ſhells were produced and depoſited. A colony of echini, or madrepores, or cornua ammonis, lived and periſhed in one period of time; in another a new colony of either ſimilar or different ſhells lived and died over the former ones, producing a ſtratum of more recent ſhell over a ſtratum of others which had began to petrify or to become marble; and thus from unknown depths to what are now the ſummits of mountains the limeſtone is diſpoſed in ſtrata of varying ſolidity and colour. Theſe have afterwards undergone variety of changes by their ſolution and depoſition from the water in which they were immerſed, or from having been expoſed to great heat under great preſſure, according to the ingenious theory of Dr. Hutton, Edinb. Tranſact. Vol. I. Sec Note XVI.

[50] 3. In moſt of the coal-countries of this iſland there are from five to ſeven beds of coal ſtratified with an equal number of beds, though of much greater thickneſs, of clay and ſandſtone, and occaſionally of iron-ores. In what manner to account for the ſtratification of theſe materials ſeems to be a problem of greater difficulty. Philoſophers have generally ſuppoſed that they have been arranged by the currents of the ſea; but conſidering their inſolubility in water, and their almoſt ſimilar ſpecific gravity, an accumulation of them in ſuch diſtinct beds from this cauſe is altogether inconceiveable, though ſome coal-countries bear marks of having been at ſome time immerſed beneath the waves and raiſed again by ſubterranean fires.

The higher and lower parts of moraſſes were neceſſarily produced at different periods of time, ſee Note XVII. and would thus originally be formed in ſtrata of different ages. For when an old wood periſhed, and produced a moraſs, many centuries would elapſe before another wood could grow and periſh, again upon the ſame ground, which would thus produce a new ſtratum of moraſs over the other, differing indeed principally in its age, and perhaps, as the timber might be different, in the proportions of its component parts.

Now if we ſuppoſe the lowermoſt ſtratum of a moraſs become ignited, like fermenting hay, (after whatever could be carried away by ſolution in water was gone,) what would happen? Certainly the inflammable part, the oil, ſulphur, or bitumen, would burn away, and be evaporated in air; and the fixed parts would be left, as clay, lime, and iron; while ſome of the calcareous earth would join with the ſiliceous acid, and produce ſand, or with the argillaceous earth, and produce marl. Thence after many centuries another bed would take fire, but with leſs degree of ignition, and with a greater body of moraſs over it, what then would happen? The bitumen and ſulphur would riſe and might become condenſed under an impervious ſtratum, which might not be ignited, and there form coal of different purities according to its degree of fluidity, which would permit ſome of the clay to ſubſide through it into the place from which it was ſublimed.

Some centuries afterwards another ſimilar proceſs might take place, and either thicken the coal-bed, or produce a new clay-bed, or marl, or ſand, or depoſit iron upon it, according to the concomitant circumſtances above mentioned.

I do not mean to contend that a few maſſes of ſome materials may not have been rolled together by currents, when the mountains were much more elevated than at preſent, and in conſequence the rivers broader and more rapid, and the ſtorms of rain and wind greater both in quantity and force. Some gravel-beds may have been thus waſhed from the mountains; and ſome white clay waſhed from moraſſes into valleys beneath them; and ſome ochres of iron diſſolved and again depoſited by water; and ſome calcareous depoſitions from water, (as the bank for inſtance on which ſtand the houſes at Matlock-bath;) but theſe are of ſmall extent or conſequence compared to the primitive rocks of granite or porpyhry which form the nucleus of the earth, or to the immenſe ſtrata of limeſtone which cruſt over the greateſt part of this granite or porphyry; or laſtly to the very extenſive beds of clay, marl, ſandſtone, coal, and iron, [51] which were probably for many millions of years the only parts of our continents and iſlands, which were then elevated above the level of the ſea, and which on that account became covered with vegetation, and thence acquired their later or ſuperincumbent ſtrata, which conſtitute, what ſome have termed, the new world.

There is another ſource of clay, and that of the fineſt kind, from decompoſed granite, this is of a ſnowy white and mixed with ſhining particles of mica, of this kind is an earth from the country of Cherokees. Other kinds are from leſs pure lavas; Mr. Ferber aſſerts that the ſulphurous ſteams from Mount Veſuvius convert the lava into clay.

‘The lavas of the antient Solfatara volcano have been undoubtedly of a vitreous nature, and theſe appear at preſent argillaceous. Some fragments of this lava are but half or at one ſide changed into clay, which either is viſcid or ductile, or hard and ſtoney. Clays by fire are deprived of their coherent quality, which cannot be reſtored to them by pulverization, nor by humectation. But the ſulphureous Solfatara ſteams reſtore it, as may be eaſily obſerved on the broken pots wherein they gather the ſal ammoniac; though very well baked and burnt at Naples they are mollified again by the acid ſteams into a viſcid clay which keeps the ſormer fire-burnt colour. Travels in Italy, p. 156.’

NOTE XXI.—ENAMELS.

THE fine bright purples or roſe colours which we ſee on china cups are not producible with any other material except gold, manganeſe indeed gives a purple but of a very different kind.

In Europe the application of gold to theſe purpoſes appears to be of modern invention. Caſſius's diſcovery of the precipitate of gold by tin, and the uſe of that precipitate for colouring glaſs and enamels, are now generally known, but though the precipitate with tin be more ſucceſſful in producing the ruby glaſs, or the colourleſs glaſs which becomes red by ſubſequent ignition, the tin probably contributing to prevent the gold from ſeparating, (which it is very liable to do during the fuſion; yet, for enamels, the precipitates made by alcaline ſalts anſwer equally well, and give a finer red, the colour produced by the tin precipitate being a bluiſh purple, but with the others a roſe red. I am informed that ſome of our beſt artiſts prefer aurum fulminans, mixing it, before it has become dry, with the white compoſition or enamel flux; when once it is divided by the other matter, it is ground with great ſafety, and without the leaſt danger of exploſion, whether moiſt or dry. The colour is remarkably improved and brought forth by long grinding, which accordingly makes an eſſential circumſtance in the proceſs.

[52] The precipitates of gold, and the colcothar or other red preparations of iron, are called tender colours. The heat muſt be no greater than is juſt ſufficient to make the enamel run upon the piece, for if greater, the colours will be deſtroyed or changed to a different kind. When the vitreous matter has juſt become fluid it ſeems as if the coloured metallic calx remained barely intermixed with it, like a coloured powder of exquiſite tenuity ſuſpended in water: but by ſtronger fire the calx is diſſolved, and metallic colours are altered by ſolution in glaſs as well as in acids or alcalies.

The Saxon mines have till very lately almoſt excluſively ſupplied the reſt of Europe with cobalt, or rather with its preparations, zaffre and ſmalt, for the exportation of the ore itſelf is there a capital crime. Hungary, Spain, Sweden, and ſome other parts of the continent, are now ſaid to afford cobalts equal to the Saxon, and ſpecimens have been diſcovered in our own iſland, both in Cornwall and in Scotland; but hitherto in no great quantity.

Calces of cobalt and of copper differ very materially from thoſe above mentioned in their application for colouring enamels. In thoſe the calx has previouſly acquired the intended colour, a colour which bears a red heat without injury, and all that remains is to fix it on the piece by a vitreous flux. But the blue colour of cobalt, and the green or bluiſh green of copper, are produced by vitrification, that is, by ſolution in the glaſs, and a ſtrong fire is neceſſary for their perfection. Theſe calces therefore, when mixed with the enamel flux, are melted in crucibles, once or oftener, and the deep coloured opake glaſs, thence reſulting, is ground into unpalpable powder, and uſed for enamel. One part of either of theſe calces is put to ten, ſixteen, or twenty parts of the flux, according to the depth of colour required. The heat of the enamel kiln is only a full red, ſuch as is marked on Mr. Wedgwood's thermometer 6 degrees. It is therefore neceſſary that the flux be ſo adjuſted as to melt in that low heat. The uſual materials are flint, or flint-glaſs, with a due proportion of red-led, or borax, or both, and ſometimes a little tin calx to give opacity.

Ka-o-lin is the name given by the Chineſe to their porcelain clay, and pe-tun-tſe to the other ingredient in their China ware. Specimens of both theſe have been brought into England, and found to agree in quality with ſome of our own materials. Kaolin is the very ſame with the clay called in Cornwall and the petuntſe is a granite ſimilar to the Corniſh moorſtone. There are differences, both in the Chineſe petuntſes, and the Engliſh moorſtones; all of them contain micaceous and quartzy particles, in greater or leſs quantity, along with feltſpat, which laſt is the eſſential ingredient for the porcelain manufactory. The only injurious material commonly found in them is iron, which diſcolours the ware in proportion to its quantity, and which our moorſtones are perhaps more frequently tainted with than the Chineſe. Very fine porcelain has been made from Engliſh materials but the nature of the manufacture renders the proceſs pre⯑carious and the profit hazardous; for the ſemivitrification, which conſtitutes porcelain, is neceſſarily accompanied with a degree of ſoftneſs, or ſemifuſion, ſo that the veſſels are liable to have their forms altered in the kiln, or to run together with any accidental augmentations of the fire. []

NOTE XXII.—PORTLAND VASE.

THE celebrated funereal vaſe, long in poſſeſſion of the Barberini family, and lately purchaſed by the Duke of Portland for a thouſand guineas, is about ten inches high and ſix in diameter in the broadeſt part. The figures are of moſt exquiſite workmanſhip in bas relief of white opake glaſs, raiſed on a ground of deep blue glaſs, which appears black except when held againſt the light. Mr. Wedgwood is of opinion from many circumſtances that the figures have been made by cutting away the external cruſt of white opake glaſs, in the manner the fineſt cameo's have been produced, and that it muſt thence have been the labour of a great many years. Some antiquarians have placed the time of its production many centuries before the chriſtian aera; as ſculpture was ſaid to have been declining in reſpect to its excellence in the time of Alexander the Great. See an account of the Barberini or Portland vaſe by M. D'Hancarville, and by Mr. Wedgwood.

Many opinions and conjectures have been publiſhed concerning the figures on this celebrated vaſe. Having carefully examined one of Mr. Wedgwood's beautiful copies of this wonderful production of art, I ſhall add one more conjecture to the number.

Mr. Wedgwood has well obſerved that it does not ſeem probable that the Portland vaſe was purpoſely made for the aſhes of any particular perſon deceaſed, becauſe many years muſt have been neceſſary for its production. Hence it may be concluded, that the ſubject of its embelliſhments is not private hiſtory but of a general nature. This ſubject appears to me to be well choſen, and the ſtory to be finely told; and that it repreſents what in antient times engaged the attention of philoſophers, poets, and heroes, I mean a part of the Eleuſinian myſteries.

Theſe myſteries were invented in Aegypt, and afterwards tranſferred to Greece, and flouriſhed more particularly at Athens, which was at the ſame time the ſeat of the fine arts. They conſiſted of ſcenical exhibitions repreſenting and inculcating the expectation of a future life after death, and on this account were encouraged by the government, inſomuch that the Athenian laws puniſhed a diſcovery of their ſecrets with death. Dr. Warburton has with great learning and ingenuity ſhewn that the deſcent of Aeneas into hell, deſcribed in the Sixth Book of Virgil, is a poetical account of the repreſen⯑tations of the future ſtate in the Eleuſinian myſteries. Divine Legation, Vol. I. p, 210.

And though ſome writers have differed in opinion from Dr. Warburton on this ſubject, becauſe Virgil has introduced ſome of his own heroes into the Elyſian fields, as Deiphobus, Palinurus, and Dido, in the ſame manner as Homer had done before him, yet it is agreed that the received notions about a future ſtate were exhibited in theſe myſteries, and as theſe poets deſcribed thoſe received notions, they may be ſaid, as far as theſe religious doctrines were concerned, to have deſcribed the myſteries.

[54] Now as theſe were emblematic exhibitions they muſt have been as well adapted to the purpoſes of ſculpture as of poetry, which indeed does not ſeem to have been un⯑common, ſince one compartment of figures in the ſheild of Aeneas repreſented the regions of Tartarus. Aen. Lib. X. The proceſſion of torches, which according to M. De St. Croix was exhibited in theſe myſteries, is ſtill to be ſeen in baſſo relievo, diſcovered by Spon and Wheler. Memoires ſur le Myſteres par De St. Croix. 1784. And it is very probable that the beautiful gem repreſenting the marriage of Cupid and Pſyche, as de⯑ſcribed by Apulcus, was originally deſcriptive of another part of the exhibitions in theſe myſteries, though afterwards it became a common ſubject of antient art. See Divine Legat. Vol. I. p. 323. What ſubject could have been imagined ſo ſublime for the ornaments of a funereal urn as the mortality of all things and their reſuſcitation? Where could the deſigner be ſupplied with emblems for this purpoſe, before the Chriſtian aera, but from the Eleuſinian myſteries?

1. The exhibitions of the myſteries were of two kinds, thoſe which the people were permitted to ſee, and thoſe which were only ſhewn to the initiated. Concerning the latter, Ariſtides calls them ‘the moſt ſhocking and moſt raviſhing repreſentations.’ And Stoboeus aſſerts that the initiation into the grand myſteries exactly reſembles death. Divine Legat. Vol. I. p. 280, and p. 272. And Virgil in his entrance to the ſhades below, amongſt other things of terrible form, mentions death. Aen. VI. This part of the exhibition ſeems to be repreſented in one of the compartments of the Portland vaſe.

Three figures of exquiſite workmanſhip are placed by the ſide of a ruined column whoſe capital is fallen off, and lies at their feet with other diſjointed ſtones, they ſit on looſe piles of ſtone beneath a tree, which has not the leaves of any evergreen of this climate, but may be ſuppoſed to be an elm, which Virgil places near the entrance of the infernal regions, and adds, that a dream was believed to dwell under every leaf of it. Aen. VI. l. 281. In the midſt of this group reclines a female figure in a dying attitude, in which extreme languor is beautifully repreſented, in her hand is an inverted torch, an antient emblem of extinguiſhed life, the elbow of the ſame arm reſting on a ſtone ſupports her as ſhe ſinks, while the other hand is raiſed and thrown over her drooping head, in ſome meaſure ſuſtaining it and gives with great art the idea of fainting laſſitude. On the right of her ſits a man, and on the left a woman, both ſupporting themſelves on their arms, as people are liable to do when they are thinking intenſely. They have their backs towards the dying figure, yet with their faces turned towards her, as if ſeriouſly contemplating her ſituation, but without ſtretching out their hands to aſſiſt her.

This central figure then appears to me to be an hieroglyphic or Eleuſinian emblem of MORTAL LIFF, that is, the lethum, or death, mentioned by Virgil amongſt the terrible things exhibited at the beginning of the myſteries. The inverted torch ſhews the figure to be emblematic, if it had been deſigned to repreſent a real perſon in the act of dying there had been no neceſſity for the expiring torch, as the dying figure alone would have been ſufficiently intelligible;—it would have been as abſurd as to have put an inverted torch into the hand of a real perſon at the time of his expiring. Beſides if this []

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[55] figure had repreſented a real dying perſon would not the other figures, or one of them at leaſt, have ſtretched out a hand to ſupport her, to have eaſed her fall among looſe ſtones, or to have ſmoothed her pillow? Theſe circumſtances evince that the figure is an emblem, and therefore could not be a repreſentation of the private hiſtory of any par⯑ticular family or event.

The man and woman on each ſide of the dying figure muſt be conſidered as emblems, both from their ſimilarity of ſituation and dreſs to the middle figure, and their being grouped along with it. Theſe I think are hieroglyphic or Eleuſinian emblems of HUMANKIND, with their backs toward the dying figure of MORTAL LIFE, unwilling to aſſociate with her, yet turning back their ſerious and attentive countenances, curious indeed to behold, yet ſorry to contemplate their latter end. Theſe figures bring ſtrongly to one's mind the Adam and Eve of ſacred writ, whom ſome have ſuppoſed to have been allegorical or hieroglyphic perſons of Aegyptian origin, but of more antient date, amongſt whom I think is Dr. Warburton. According to this opinion Adam and Eve were the names of two hieroglyphic figures repreſenting the early ſtate of mankind; Abel was the name of an hieroglyphic figure repreſenting the age of paſturage, and Cain the name of another hieroglyphic ſymbol repreſenting the age of agriculture, at which time the uſes of iron were diſcovered. And as the people who cultivated the earth and built houſes would increaſe in numbers much faſter by their greater production of food, they would readily conquer or deſtroy the people who were ſuſtained by paſturage, which was typified by Cain ſlaying Abel.

2. On the other compartment of this celebrated vaſe is exhibited an emblem of immortality, the repreſentation of which was well known to conſtitute a very principal part of the ſhews at the Eleuſinian myſteries, as Dr. Warburton has proved by variety of authority. The habitation of ſpirits or ghoſts after death was ſuppoſed by the antients to be placed beneath the earth, where Pluto reigned, and diſpenſed rewards or puniſhments. Hence the firſt figure in this group is of the MANES or GHOST, who having paſſed through an open portal is deſcending into a duſky region, pointing his toe with timid and unſteady ſtep, feeling as it were his way in the gloom. This portal Aeneas enters, which is deſcribed by Virgil,—patet atri janua ditis, Aen. VI. l. 126; as well as the eaſy deſcent,—facilis deſcenſus Averni. Ib. The darkneſs at the entrance to the ſhades is humorouſly deſcribed by Lucian. Div. Legat. Vol. I. p. 241. And the horror of the gates of hell was in the time of Homer become a proverb; Achilles ſays to Ulyſſes, "I hate a liar worſe than the gates of hell;" the ſame expreſſion is uſed in Iſaiah, ch. xxxviii. v. 10. The MANES or GHOST appears lingering and fearful, and wiſhes to drag after him a part of his mortal garment, which however adheres to the ſide of the portal through which he has paſſed. The beauty of this allegory would have been expreſſed by Mr. Pope, by "We feel the ruling paſſion ſtrong in death."

A little lower down in the group the manes or ghoſt is received by a beautiful female, a ſymbol of IMMORTAL LIFE. This is evinced by her fondling between her knees a large and playful ſerpent, which from its annually renewing its external ſkin has from great antiquity, even as early as the fable of Prometheus, been eſteemed an emblem of [56] renovated youth. The ſtory of the ſerpent acquiring immortal life from the aſs of Prometheus, who carried it on his back, is told in Bacon's Works, Vol. V. p. 462. Quarto edit. Lond. 1778. For a ſimilar purpoſe a ſerpent was wrapped round the large hieroglyphic egg in the temple of Dioſcuri, as an emblem of the renewal of life from a ſtate of death. Bryant's Mythology, Vol II. p. 359. ſec. edit. On this account alſo the ſerpent was an attendant on Aeſculapius, which ſeems to have been the name of the hieroglyphic figure of medicine. This ſerpent ſhews this figure to be an emblem, as the torch ſhewed the central figure of the other compartment to be an emblem, hence they agreeably correſpond, and explain each other, one repreſenting MORTAL LIFE, and the other IMMORTAL LIFE.

This emblematic figure of immortal life ſits down with her feet towards the figure of Pluto, but, turning back her face towards the timid ghoſt, ſhe ſtretches forth her hand, and taking hold of his elbow, ſupports his tottering ſteps, as well as encourages him to advance, both which circumſtances are thus with wonderful ingenuity brought to the eye. At the ſame time the ſpirit looſely lays his hand upon her arm, as one walking in the dark would naturally do for the greater certainty of following his conductreſs, while the general part of the ſymbol of IMMORTAL LIFE, being turned toward the figure of Pluto, ſhews that ſhe is leading the phantom to his realms.

In the Pamphili gardens at Rome, Perſeus in aſſiſting Andromeda to deſcend from the rock takes hold of her elbow to ſteady or ſupport her ſtep, and ſhe lays her hand looſely on his arm as in this figure. Admir. Roman. Antiq.

The figure of PLUTO can not be miſtaken, as is agreed by moſt of the writers who have mentioned this vaſe; his griſley beard, and his having one foot buried in the earth, denotes the infernal monarch. He is placed at the loweſt part of the group, and reſting his chin on his hand, and his arm upon his knee, receives the ſtranger-ſpirit with inquiſitive attention; it was before obſerved that when people think attentively they naturally reſt their bodies in ſome eaſy attitude, that more animal power may be employed on the thinking faculty. In this group of figures there is great art ſhewn in giving an idea of a deſcending plain, viz. from earth to Elyſium, and yet all the figures are in reality on an horizontal one. This wonderful deception is produced firſt by the deſcend⯑ing ſtep of the manes or ghoſt; ſecondly, by the arm of the ſitting figure of immortal life being raiſed up to receive him as he deſcends; and laſtly, by Pluto having one foot ſunk into the earth.

There is yet another figure which is concerned in conducting the manes or ghoſt to the realms of Pluto, and this is LOVE. He precedes the deſcending ſpirit on expanded wings, lights him with his torch, and turning back his beautiful countenance beckons him to advance. The antient God of love was of much higher dignity than the modern Cupid. He was the firſt that came out of the great egg of night, (Heſiod. Theog. V. CXX. Bryant's Mythol. Vol. II. p. 348.) and is ſaid to poſſeſs the keys of the ſky, ſea, and earth. As he therefore led the way into this life, he ſeems to conſtitute a proper emblem for leading the way to a ſuture life. See Bacon's works. Vol. I. p. 568. and Vol. III. p. 582. Quarto edit.

[57] The introduction of love into this part of the myſteries requires a little further expla⯑nation. The Pſyche of the Aegyptians was one of their moſt favourite emblems, and repreſented the ſoul, or a future life; it was originally no other than the aurelia, or butterfly, but in after times was repreſented by a lovely female child with the beautiful wings of that inſect. The aurelia, after its firſt ſtage as an eruca or caterpillar, lies for a ſeaſon in a manner dead, and is incloſed in a ſort of coffin, in this ſtate of darkneſs it remains all the winter, but at the return of ſpring it burſts its bonds and comes out with new life, and in the moſt beautiful attire. The Aegyptians thought this a very proper picture of the ſoul of man, and of the immortality to which it aſpired. But as this was all owing to divine Love, of which EROS was an emblem, we find this perſon frequently introduced as a concomitant of the ſoul in general or Pſyche. (Bryant's Mythol. Vol. II. p. 386.) EROS, or divine Love, is for the ſame reaſon a proper attendant on the manes or ſoul after death, and much contributes to tell the ſtory, that is, to ſhew that a ſoul or manes is deſigned by the deſcending figure. From this figure of Love M. D'Hancarville imagines that Orpheus and Eurydice are typified under the figure of the manes and immortal life as above deſcribed. It may be ſufficient to anſwer, firſt, that Orpheus is always repreſented with a lyre, of which there are prints of four different gems in Spence's Polymetis, and Virgil ſo deſcribes him, Aen. VI. cytharâ. fretus. And ſecondly, that it is abſurd to ſuppoſe that Eurydice was fondling and playing with a ſerpent that had ſlain her. Add to this that Love ſeems to have been an inhabitant of the infernal regions, as exhibited in the myſteries, for Claudian, who treats more openly of the Eleuſinian myſteries, when they were held in leſs veneration, invokes the deities to diſcloſe to him their ſecrets, and amongſt other things by what torch Love ſoftens Pluto.

In this compartment there are two trees, whoſe branches ſpread over the figures, one of them has ſmoother leaves like ſome evergreens, and might thence be ſuppoſed to have ſome alluſion to immortality, but they may perhaps have been deſigned only as ornaments, or to relieve the figures, or becauſe it was in groves, where theſe myſteries were originally celebrated. Thus Homer ſpeaks of the woods of Proſerpine, and mentions many trees in Tartarus, as preſenting their fruits to Tantalus; Virgil ſpeaks of the pleaſant groves of Elyſium; and in Spence's Polymetis there are prints of two antient gems, one of Orpheus charming Cerberus with his lyre, and the other of Hercules binding him in a cord, each of them ſtanding by a tree. Polymet. p. 284. As however theſe trees have all different foliage ſo clearly marked by the artiſt, they may have had ſpecific meanings in the exhibitions of the myſteries, which have not reached poſterity, of this kind ſeem to have been the tree of knowledge of good and evil, and the tree of life, in ſacred writ, both which muſt have been emblematic or allegorical. The maſks, [58] hanging to the handles of the vaſe, ſeem to indicate that there is a concealed meaning in the figures beſides their general appearance. And the prieſteſs at the bottom, which I come now to deſcribe, ſeems to ſhew this concealed meaning to be of the ſacred or Eleuſinian kind.

3. The figure on the bottom of the vaſe is on a larger ſcale than the others, and leſs finely finiſhed, and leſs elevated, and as this bottom part was afterwards cemented to the upper part, it might be executed by another artiſt for the ſake of expedition, but there ſeems no reaſon to ſuppoſe that it was not originally deſigned for the upper part of it as ſome have conjectured. As the myſteries of Ceres were celebrated by female prieſts, for Porphyrius ſays the antients called the prieſteſſes of Ceres, Meliſſai, or bees, which were emblems of chaſtity. Div. Leg. Vol. I. p. 235. And as, in his Satire againſt the ſex, Juvenal ſays, that few women are worthy to be prieſteſſes of Ceres. Sat. VI. the figure at the bottom of the vaſe would ſeem to repreſent a PRIESTESS or HIEROPHANT, whoſe office it was to introduce the initiated, and point out to them, and explain the exhibitions in the myſteries, and to exclude the uninitiated, calling out to them, " Far, far retire, ye profane!" and to guard the ſecrets of the temple. Thus the introductory hymn ſung by the hierophant, according to Euſebius, begins, "I will declare a ſecret to the initiated, but let the doors be ſhut againſt the profane." Div. Leg. Vol. I. p. 177. The prieſteſs or hierophant appears in this figure with a cloſe hood, and dreſſed in linen, which ſits cloſe about her; except a light cloak, which flutters in the wind. Wool, as taken from ſlaughtered animals, was eſteemed profane by the prieſts of Aegypt, who were always dreſſed in linen. Apuleus, p. 64. Div. Leg. Vol. I. p. 318. Thus Eli made for Samuel a linen ephod. Samuel i. 3.

Secrecy was the foundation on which all myſteries reſted, when publicly known they ceaſed to be myſteries; hence a diſcovery of them was not only puniſhed with death by the Athenian law; but in other countries a diſgrace attended the breach of a ſolemn oath. The prieſteſs in the figure before us has her finger pointing to her lips as an emblem of ſilence. There is a figure of Harpocrates, who was of Aegyptian origin, the ſame as Orus, with the lotus on his head, and with his finger pointing to his lips not preſſed upon them, in Bryant's Mythol. Vol. II. p. 398, and another female figure ſtanding on a lotus, as if juſt riſen from the Nile, with her finger in the ſame attitude, theſe ſeem to have been repreſentations or emblems of male and female prieſts of the ſecret myſteries. As theſe ſort of emblems were frequently changed by artiſts for their more elegant exhibition, it is poſſible the foliage over the head of this figure may bear ſome analogy to the lotus above mentioned.

This figure of ſecrecy ſeems to be here placed, with great ingenuity, as a caution to the initiated, who might underſtand the meaning of the emblems round the vaſe, not to divulge it. And this circumſtance ſeems to account for there being no written ex⯑planation extant, and no tradition concerning theſe beautiful figures handed down to us along with them.

Another explanation of this figure at the bottom of the vaſe would ſeem to confirm the idea that the baſſo relievos round its ſides are repreſentations of a part of the []

[59] myſteries, I mean that it is the head of ATIS. Lucian ſays that Atis was a young man of Phrygia, of uncommon beauty, that he dedicated a temple in Syria to Rhea, or Cybele, and firſt taught her myſteries to the Lydians, Phrygians, and Samothracians, which myſteries he brought from India. He was afterwards made an eunuch by Rhea, and lived like a woman, and aſſumed a feminine habit, and in that garb went over the world teaching her ceremonies and myſteries. Dict. par M. Danet, art. Atis. As this figure is covered with clothes, while thoſe on the ſides of the vaſe are naked, and has a Phrygian cap on the head, and as the form and features are ſo ſoft, that it is difficult to ſay whether it be a male or female figure, there is reaſon to conclude, 1. that it has reference to ſome particular perſon of ſome particular country; 2. that this perſon is Atis, the firſt great hierophant, or teacher of myſteries, to whom M. De la Chauſſe ſays the figure itſelf bears a reſemblance. Muſeo. Capitol. Tom. IV. p. 402.

In the Muſeum Etruſcum, Vol. I. plate 96, there is the head of Atis with feminine features, clothed with a Phrygian cap, and riſing from very broad foliage, placed on a kind of term ſupported by the paw of a lion. Goreus in his explanation of the figure ſays that it is placed on a lion's foot becauſe that animal was ſacred to Cybele, and that it riſes from very broad leaves becauſe after he became an eunuch he determined to dwell in the groves. Thus the foliage, as well as the cap and feminine features, confirm the idea of this figure at the bottom of the vaſe repreſenting the head of Atis the firſt great hierophant, and that the figures on the ſides of the vaſe are emblems from the antient myſteries.

I beg leave to add that it does not appear to have been uncommon amongſt the antients to put allegorical figures on funeral vaſes. In the Pamphili palace at Rome there is an elaborate repreſentation of Life and of Death, on an antient ſarcophagus. In the firſt Prometheus is repreſented making man, and Minerva is placing a butterfly, or the foul, upon his head. In the other compartment Love extinguiſhes his torch in the boſom of the dying figure, and is receiving the butterfly, or Pſyche, from him, with a great number of complicated emblematic figures grouped in very bad taſte. Admir. Roman. Antiq.

NOTE XXIII.—COAL.

TO elucidate the formation of coal-beds I ſhall here deſcribe a fountain of foſſil tar, or petroleum, diſcovered lately near Colebrook Dale in Shropſhire, the particulars of which were ſent me by Dr. Robert Darwin of Shrewſbury.

About a mile and a half below the celebrated iron-bridge, conſtructed by the late Mr. DARBY near Colebrook Dale, on the eaſt ſide of the river Severn, as the workmen in October 1786 were making a ſubterranean canal into the mountain, for the more eaſy acquiſition and conveyance of the coals which lie under it, they found an oozing of liquid bitumen, or petroleum; and as they proceeded further cut through ſmall cavities of different ſizes from which the bitumen iſſued. From ten to fifteen barrels of this foſſil tar, each barrel containing thirty-two gallons, were at firſt collected in a day, which has ſince however gradually diminiſhed in quantity, ſo that at preſent the product is about ſeven barrels in fourteen days.

The mountain, into which this canal enters, conſiſts of ſiliceous ſand, in which however a few marine productions, apparently in their recent ſtate, have been found, and are now in the poſſeſſion of Mr. WILLIAM REYNOLDS of Ketly Bank. About three hundred yards from the entrance into the mountain, and about twenty-eight yards below the ſurface of it, the tar is found oozing from the ſand-rock above into the top and ſides of the canal.

Beneath the level of this canal a ſhaft has been ſunk through a grey argillaceous ſubſtance, called in this country clunch, which is ſaid to be a pretty certain indication of coal; beneath this lies a ſtratum of coal, about two or three inches thick, of an inferior kind, yielding little flame in burning, and leaving much aſhes; below this is a rock of a harder texture; and beneath this are found coals of an excellent quality; for the purpoſe of procuring which with greater facility the canal, or horizontal aperture, is now making into the mountain. July, 1788.

Beneath theſe coals in ſome places is found ſalt water, in other parts of the adjacent country there are beds of iron-ſtone, which alſo contain ſome bitumen in a leſs fluid ſtate, and which are about on a level with the new canal, into which the foſſil tar oozes, as above deſcribed.

There are many intereſting circumſtances attending the ſituation and accompaniments of this fountain of foſſil tar, tending to develop the manner of its production. 1. As the canal paſſing into the mountain runs over the beds of coals, and under the reſervoir of petroleum, it appears that a natural diſtillation of this foſſil in the bowels of the earth muſt have taken place at ſome early period of the world, ſimilar to the artificial diſtillation [61] of coal, which has many years been carried on in this place on a ſmaller ſcale above ground. When this reſervoir of petroleum was cut into, the ſlowneſs of its exſudation into the canal was not only owing to its viſcidity, but to the preſſure of the atmoſphere, or to the neceſſity there was that air ſhould at the ſame time inſinuate itſelf into the ſmall cavities from which the petroleum deſcended. The exiſtence of ſuch a diſtillation at ſome antient time is confirmed by the thin ſtratum of coal beneath the canal, (which covers the hard rock,) having been deprived of its foſſil oil, ſo as to burn without flame, and thus to have become a natural coak, or foſſil charcoal, while the petroleum diſtilled from it is found in the cavities of the rock above it.

There are appearances in other places, which favour this idea of the natural diſtillation of petroleum, thus at Matlock in Derbyſhire a hard bitumen is found adhering to the ſpar in the clefts of the lime-rocks in the form of round drops about the ſize of peas; which could perhaps only be depoſited there in that form by ſublimation.

2. The ſecond deduction, which offers itſelf, is, that theſe beds of coal have been expoſed to a conſiderable degree of heat, ſince the petroleum above could not be ſeparated, as far as we know, by any other means, and that the good quality of the coals beneath the hard rock was owing to the impermeability of this rock to the bituminous vapour, and to its preſſure being too great to permit its being removed by the elaſticity of that vapour. Thus from the degree of heat, the degree of preſſure, and the permeability of the ſuperincumbent ſtrata, many of the phenomena attending coal-beds receive an eaſy explanation, which much accords with the ingenious theory of the earth by Dr. Hutton. Trans. of Edinb. Vol. I.

In ſome coal works the fuſion of the ſtrata of coal has been ſo ſlight, that there remains the appearance of ligneus fibres, and the impreſſion of leaves, as at Bovey near Exeter, and even ſeeds of vegetables, of which I have had ſpecimens from the collieries near Poleſworth in Warwickſhire. In ſome, where the heat was not very intenſe and the incumbent ſtratum not permeable to vapour, the foſſil oil has only riſen to the upper part of the coal-bed, and has rendered that much more inflammable than the lower parts of it, as in the collieries near Beaudeſert, the ſeat of the EARL OF UXBRIDGE in Staffordſhire, where the upper ſtratum is a perfect cannel, or candle-coal, and the lower one of an inferior quality. Over the coal-beds near Sir H. HARPUR's houſe in Derbyſhire a thin lamina of aſphaltum is found in ſome places near the ſurface of the earth, which would ſeem to be from a diſtillation of petroleum from the coals below, the more fluid part of which had in proceſs of time exhaled, or been conſolidated by its abſorption of air. In other coal-works the upper part of the ſtratum is of a worſe kind than the lower one, as at Alfreton and Denbigh in Derbyſhire, owing to the ſuper⯑cumbent ſtratum having permitted the exhalation of a great part of the petroleum; whilſt at Widdrington in Northumberland there is firſt a ſeam of coal about ſix inches thick of no value, which lies under about four fathom of clay, beneath this is a white freeſtone, then a hard ſtone, which the workmen there call a whin, then two fathoms of clay, then another white ſtone, and under that a vein of coals three feet nine inches [62] thick, of a ſimilar nature to the Newcaſtle coal. Phil. Trans. Abridg. Vol. VI. plate II. p. 192. The ſimilitude between the circumſtances of this colliery, and of the coal beneath the fountain of tar above deſcribed, renders it highly probable that this upper thin ſeam of coal has ſuffered a ſimilar diſtillation, and that the inflammable part of it had either been received into the clay above in the form of ſulphur, which when burnt in the open air would produce alum; or had been diſſipated for want of a receiver, where it could be condenſed. The former opinion is perhaps in this cafe more probable as in ſome other coal-beds, of which I have procured accounts, the ſurface of the coal beneath clunch or clay is of an inferior quality, as at Weſt Hallum in Nottinghamſhire. The clunch probably from hence acquires its inflammable part, which on calcination becomes vitriolic acid. I gathered pieces of clunch converted partially into alum at a colliery near Bilſton, where the ground was ſtill on fire a few years ago.

The heat, which has thus pervaded the beds of moraſs, ſeems to have been the effect of the fermentation of their vegetable materials; as new hay ſometimes takes fire even in ſuch very ſmall maſſes from the ſugar it contains, and ſeems hence not to have been attended with any expulſion of lava, like the deeper craters of volcanos ſituated in beds of granite.

3. The marine ſhells found in the looſe ſand-rock above this reſervoir of petroleum, and the coal-beds beneath it, together with the exiſtence of ſea-ſalt beneath theſe coals, prove that theſe coal beds have been at the bottom of the ſea, during ſome remote period of time, and were afterwards raiſed into their preſent ſituation by ſubterraneous expan⯑ſions of vapour. This doctrine is further ſupported by the marks of violence, which ſome coal-beds received at the time they were raiſed out of the ſea, as in the collieries at Mendip in Somerſetſhire. In theſe there are ſeven ſtrata of coals, equitant upon each other, with beds of clay and ſtone intervening; amongſt which clay are found ſhells and fern branches. In one part of this hill the ſtrata are diſjoined, and a quantity of heterogeneous ſubſtances fill up the chaſm which diſjoins them, on one ſide of this chaſm the ſeven ſtrata of coal are ſeen correſponding in reſpect to their reciprocal thick⯑neſs and goodneſs with the ſeven ſtrata on the other ſide of the cavity, except that they have been elevated ſeveral yards higher. Phil. Tranſ. No. 360. abridg. Vol. V. p. 237.

The cracks in the coal-bed near Ticknall in Derbyſhire, and in the ſand-ſtone rock over it, in both of which ſpecimens of lead-ore and ſpar are found, confirm this opinion of their having been forcibly raiſed up by ſubterraneous fires. Over the colliery at Brown-hills near Lichfield, there is a ſtratum of gravel on the ſurface of the ground; which may be adduced as another proof to ſhew that thoſe coals had ſome time been beneath the ſea, or the bed of a river. Nevertheleſs, theſe arguments only apply to the collieries above mentioned, which are few compared with thoſe which bear no marks of having been immerſed in the ſea.

On the other hand the production of coals from moraſſes, as deſcribed in note XX. is evinced from the vegetable matters frequently found in them, and in the ſtrata over them; as fern-leaves in nodules of iron-ore, and from the bog-ſhells or freſh water [63] muſcles ſometimes found over them, of both which I have what I believe to be ſpeci⯑mens; and is further proved from ſome parts of theſe beds being only in part trans⯑formed to coal; and the other part ſtill retaining not only the form, but ſome of the properties of wood; ſpecimens of which are not unfrequent in the cabinets of the curious, procured from Loch Neigh in Ireland, from Bovey near Exeter, and other places; and from a famous cavern called the Temple of the Devil, near the town of Altorf in Fran⯑conia, at the foot of a mountain covered with pine and ſavine, in which are found large coals reſembling trees of ebony; which are ſo far mineralized as to be heavy and com⯑pact; and ſo to effloreſce with pyrites in ſome parts as to crumble to pieces; yet from other parts white aſhes are produced on calcination, from which fixed alcali is procured; which evinces their vegetable origin. (Dict. Raiſonné, art. Charbon.) To theſe may be added another argument from the oil which is diſtilled from coals, and which is analogous to vegetable oil, and does not exiſt in any bodies truly mineral. Keir's Chemical Dictionary, art. Bitumen.

Whence it would appear, that though moſt collieries with their attendant ſtrata of clay, ſand-ſtone, and iron, were formed on the places where the vegetables grew, from which they had their origin; yet that other collections of vegetable matter were waſhed down from eminences by currents of waters into the beds of rivers, or the neighbouring ſeas, and were there accumulated at different periods of time, and under⯑went a great degree of heat from their fermentation, in the ſame manner as thoſe beds of moraſs which had continued on the plains where they were produced. And that by this fermentation many of them had been raiſed from the ocean with ſand and ſea-ſhells over them; and others from the beds of rivers with accumulations of gravel upon them.

4. For the purpoſe of bringing this hiſtory of the products of moraſſes more diſtinctly to the eye of the reader, I ſhall here ſubjoin two or three accounts of ſinking or boring for coals, out of above twenty which I have procured from various places, though the terms are not very intelligible, being the language of the overſeers of coal-works.

1. Whitfield mine near the Pottery in Staffordſhire. Soil 1 foot. brick-clay 3 feet. ſhale 4. metal which is hard brown and falls in the weather 42. coal 3. warrant clay 6. brown gritſtone 36. coal 3 ½. warrant clay 3 ½. baſs and metal 53 ½. hardſtone 4. ſhaly baſs 1 ½. coal 4. warrant clay, depth unknown. in all about 55 yards.

2. Coal-mine at Alfreton in Derbyſhire. Soil and clay 7 feet. fragments of ſtone 9. bind 13. ſtone 6. bind 34. ſtone 5. bind 2. ſtone 2. bind 10. coal 1 ½. bind 1 ½. ſtone 37. bind 7. ſoft coal 3. bind 3. ſtone 20. bind 16. coal 7 ½. in all about 61 yards.

3. A baſſet coal-mine at Woolartan in Nottinghamſhire. Sand and gravel 6 feet. bind 21. ſtone 10. ſmut or effete coal 1. clunch 4. bind 21. ſtone 18. bind 18. ſtone⯑bind 15. ſoft coal 2. clunch and bind 21. coal 7. in all about 48 yards.

4. Coal-mine at Weſt-Hallam in Nottinghamſhire. Soil and clay 7 feet. bind 48. ſmut 1 ½. clunch 4. bind 3. ſtone 2. bind 1. ſtone 1. bind 3. ſtone 1. bind 16. ſhale 2. bind 12. ſhale 3. clunch, ſtone, and a bed of cank 54. ſoft coal 4. clay and dun 1. ſoft coal 4 ½. clunch and bind 21. coal 1. broad bind 26. hard coal 6. in all about 74 yards.

[64] As theſe ſtrata generally lie inclined, I ſuppoſe parallel with the limeſtone on which they reſt, the upper edges of them all come out to day, which is termed baſſetting; when the whole maſs was ignited by its fermentation, it is probable that the inflam⯑mable part of ſome ſtrata might thus more eaſily eſcape than of others in the form of vapour; as dews are known to ſlide between ſuch ſtrata in the production of ſprings; which accounts for ſome coal-beds being ſo much worſe than others. See note XX.

NOTE XXIV.—GRANITE.

THE loweſt ſtratum of the earth which human labour has arrived to, is granite; and of this likewiſe conſiſts the higheſt mountains of the world. It is known under variety of names according to ſome difference in its appearance or compoſition, but is now generally conſidered by philoſophers as a ſpecies of lava; if it contains quartz, felt⯑ſpat, and mica in diſtinct cryſtals, it is called granite; which is found in Cornwall in rocks; and in looſe ſtones in the gravel near Drayton in Shropſhire, in the road towards Newcaſtle. If theſe parts of the compoſition be leſs diſtinct, or if only two of them be viſible to the eye, it is termed porphyry, trap, whinſtone, moorſtone, ſlate. And if it appears in a regular angular form, it is called baſaltes. The affinity of theſe bodies has lately been further well eſtabliſhed by Dr. Beddoes in the Phil. Tranſ. Vol. LXXX.

Theſe are all eſteemed to have been volcanic productions that have undergone different degrees of heat; it is well known that in Papin's digeſter water may be made red hot by confinement, and will then diſſolve many bodies which otherwiſe are little or not at all acted upon by it. From hence it may be conceived, that under immenſe preſſure of ſuperincumbent materials, and by great heat, theſe maſſes of lava may have undergone a kind of aqueous ſolution, without any tendency to vitrification, and might thence have a power of cryſtallization, whence all the varieties above mentioned from the different proportion of the materials, or the different degrees of heat they may have undergone in this aqueous ſolution. And that the uniformity of the mixture of the original earths, as of lime, argil, ſilex, magneſia, and barytes, which they contain, was owing to their boiling together a longer or ſhorter time before their elevation into mountains. See note XIX. art. 8.

The ſeat of volcanos ſeems to be principally, if not entirely, in theſe ſtrata of granite; as many of them are ſituated on granite mountains, and throw up from time to time ſheets of lava which run down over the preceeding ſtrata from the ſame origin; and in this they ſeem to differ from the heat which has ſeparated the clay, coal, and ſand in moraſſes, which would appear to have riſen from a kind of fermentation, and thus to have pervaded the whole maſs without any expuition of lava.

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[65] All the lavas from Veſuvius contain one fourth part of iron, (Kirwan's Min.) and all the five primitive earths, viz. calcareous, argillaceous, ſiliceous, barytic, and magneſian earths, which are alſo evidently produced now daily from the recrements of animal and vegetable bodies. What is to be thence concluded? Has the granite ſtratum in very antient times been produced like the preſent calcareous and ſiliceous maſſes, according to the ingenious theory of Dr. Hutton, who ſays new continents are now forming at the bottom of the ſea to riſe in their turn, and that thus the terraqueous globe has been, and will be, eternal? Or ſhall we ſuppoſe that this internal heated maſs of granite, which forms the nucleus of the earth, was a part of the body of the ſun before it was ſeparated by an exploſion? Or was the fun originally a planet, inhabited like ours, and a ſatellite to ſome other greater fun, which has long been extinguiſhed by diffuſion of its light, and around which the preſent ſun continues to revolve, according to a conjecture of the celebrated Mr. Herſchell, and which conveys to the mind a moſt ſublime idea of the progreſſive and increaſing excellence of the works of the Creator of all things?

For the more eaſy comprehenſion of the facts and conjectures concerning the ſituation and production of the various ſtrata of the earth, I ſhall here ſubjoin a ſuppoſed ſection of the globe, but without any attempt to give the proportions of the parts, or the number of them, but only their reſpective ſituation over each other, and a geological recapitulation.

NOTE XXV.—EVAPORATION.

1. THE atmoſphere will diſſolve a certain quantity of moiſture as a chemical men⯑ſtruum, even when it is much below the freezing point, as appears from the diminution of ice ſuſpended in froſty air, but a much greater quantity of water is evaporated and ſuſpended in the air by means of heat, which is perhaps the univerſal cauſe of fluidity, for water is known to boil with leſs heat in vacuo, which is a proof that it will evaporate faſter in vacuo, and that the air therefore rather hinders than promotes its evaporation in higher degrees of heat. The quick evaporation occaſioned in vacuo by a ſmall degree of heat is agreeably ſeen in what is termed a pulſe-glaſs, which conſiſts of an exhauſted tube of glaſs with a bulb at each end of it and with about two thirds of the cavity filled with alcohol, in which the ſpirit is inſtantly ſeen to boil by the heat of the finger-end applied on a bubble of ſteam in the lower bulb, and is condenſed again in the upper bulb by the leaſt conceivable comparative coldneſs.

2. Another circumſtance evincing that heat is the principal cauſe of evaporation is that at the time of water being converted into ſteam, a great quantity of heat is taken away from the neighbouring bodies. If a thermometer be repeatedly dipped in ether, or in rectified ſpirit of wine, and expoſed to a blaſt of air, to expedite the evaporation by perpetually removing the ſaturated air from it, the thermometer will preſently ſink below freezing. This warmth, taken from the ambient bodies at the time of evaporation by the ſteam, is again given out when the ſteam is condenſed into water. Hence the water in a worm-tub during diſtillation ſo ſoon becomes hot; and hence the warmth ac⯑companying the deſcent of rain in cold weather.

3. The third circumſtance, ſhewing that heat is the principal cauſe of evaporation, is, that ſome of the ſteam becomes again condenſed when any part of the heat is withdrawn. Thus when warmer ſouth-weſt winds replete with moiſture ſucceed the colder north⯑eaſt winds all bodies that are denſe and ſubſtantial, as ſtone walls, brick floors, &c. abſorb ſome of the heat from the paſſing air, and its moiſture becomes precipitated on them, while the north-eaſt winds become warmer on their arrival in this latitude, and are thence diſpoſed to take up more moiſture, and are termed drying winds.

4. Heat ſeems to be the principal cauſe of the ſolution of many other bodies, as common ſalt, or blue vitriol diſſolved in water, which when expoſed to ſevere cold are precipitated, or carried, to the part of the water laſt frozen; this I obſerved in a phial filled with a ſolution of blue vitriol which was frozen; the phial was burſt, the ice [68] thawed, and a blue column of cupreous vitriol was left ſtanding upright on the bottom of the broken glaſs, as deſcribed in note XIX.

II. Hence water may either be diſſolved in air, and may then be called an aerial ſolution of water; or it may be diſſolved in the fluid matter of heat, according to the theory of M. Lavoiſier, and may then be called ſteam. In the former caſe it is probable there are many other vapours which may precipitate it, as marine acid gas, or fluor acid gas. So alcaline gas and acid gas diſſolved in air precipitate each other, nitrous gas pre⯑cipitates vital air from its azote, and inflammable gas mixed with vital air ignited by an electric ſpark either produces or precipitates the water in both of them. Are there any ſubtle exhalations occaſionally diffuſed in the atmoſphere which may thus cauſe rain?

1. But as water is perhaps many hundred times more ſoluble in the fluid matter of heat than in air, I ſuppoſe the education of this heat, by whatever means it is occaſioned, is the principal cauſe of devaporation. Thus if a region of air is brought from a warmer climate, as the S. W. winds, it becomes cooled by its contact with the earth in this latitude, and parts with ſo much of its moiſture as was diſſolved in the quantity of calorique, or heat, which it now looſes, but retains that part which was ſuſpended by its attraction to the particles of air, or by aerial ſolution, even in the moſt ſevere froſts.

2. A ſecond immediate cauſe of rain is a ſtream of N. E. wind deſcending from a ſuperior current of air, and mixing with the warmer S. W. wind below; or the reverſe of this, viz. a ſuperior current of S. W. wind mixing with an inferior one of N. E. wind; in both theſe caſes the whole heaven becomes inſtantly clouded, and the moiſture contained in the S. W. current is precipitated. This cauſe of devaporation has been ingeniouſly explained by Dr. Hutton in the Tranſact. of Edinburgh, Vol. I. and ſeems to ariſe from this circumſtance; the particles of air of the N. E. wind educe part of the heat from the S. W. wind, and therefore the water which was diſſolved by that quantity of heat is precipitated; all the other part of the water, which was ſuſpended by its attraction to the particles of air, or diſſolved in the remainder of the heat, continues unprecipitated.

3. A third method by which a region of air becomes cooled, and in conſequence depoſits much of its moiſture, is from the mechanical expanſion of air, when part of the preſſure is taken off. In this caſe the expanded air becomes capable of receiving or attracting more of the matter of heat into its interſtices, and the vapour, which was previouſly diſſolved in this heat, is depoſited, as is ſeen in the receiver of an air-pump, which becomes dewy, as the air within becomes expanded by the education of part of it. See note VII. Hence when the mercury in the barometer ſinks without a change of the wind the air generally becomes colder. See note VII. on Elementary Heat. And it is probably from the varying preſſure of the incumbent air that in ſummer days ſmall black clouds are often thus ſuddenly produced, and again ſoon vaniſh. See a paper in Philos. Tranſ. Vol. LXXVIII. intitled Frigorific Experiments on the Mechanical Ex⯑panſion of Air.

[69] 4. Another portion of atmoſpheric water may poſſibly be held in ſolution by the electric fluid, ſince in thunder ſtorms a precipitation of the water ſeems to be either the cauſe or the conſequence of the eduction of the electricity. But it appears more pro⯑bable that the water is condenſed into clouds by the eduction of its heat, and that then the ſurplus of electricity prevents their coaleſcence into larger drops, which immediately ſucceeds the departure of the lightning.

5. The immediate cauſe why the barometer ſinks before rain is, firſt, becauſe a region of warm air, brought to us in the place of the cold air which it had diſplaced, muſt weigh lighter, both ſpecifically and abſolutely, if the height of the warm atmoſphere be ſuppoſed to be equal to that or the preceeding cold one. And ſecondly, after the drops of rain begin to fall in any column of air, that column becomes lighter, the falling drops only adding to the prſſure of the air in proportion to the reſiſtance which they meet with in paſſing through that fluid.

If we could ſuppoſe water to be diſſolved in air without heat, or in very low degrees of heat, I ſuppoſe the air would become heavier, as happens in many chemical ſolution, but if water diſſolved in the matter of heat, or calorique, be mixed with an aerial ſolutions, of water, there can be no doubt but an atmoſphere conſiſting of ſuch a mixture muſt become lighter in proportion to the quantity of calorique. On the ſame circumſtance depends the viſible vapour produced from the breath of animals in cold weather, or from a boiling kettle; the particles of cold air, with which it is mixed, ſteal a part of its heat, and become themſelves raiſed in temperature, whence part of the water is precipitated in viſible vapour, which, if in great quantity ſinks to the ground; if in ſmall quantity, and the ſurrounding air is not previouſly ſaturated, it ſpreads itſelf till it becomes again diſſolved.

NOTE XXVI.—SPRINGS.

THE ſurface of the earth conſiſts of ſtrata many of which were formed originally beneath the ſea, the mountains were afterwards forced up by ſubterraneous fires, as appears from the fillures in the rocks of which they conſiſt the quantity of volcanic productions all over the world, and the numerous remains of craters of volcanos in mountainous countries. Hence the ſtrata which compoſe the ſides of mountains lie ſlanting downwards, and one or two or more of the external ſtrata not reaching to the ſummit when the mountain was raiſed up, the ſecond or third ſtratum or a more inferior one is there expoſed to day; this may be well repreſented by forceably thruſting a blunt inſtrument through ſeveral ſheets of paper, a bur will ſtand up with the lowermoſt ſheet ſtanding higheſt in the center of it. On this uppermoſt ſtratum, which is colder as it is more elevated, the dews are condenſed in large quantities; and ſliding down paſs under the firſt or ſecond or third ſtratum which compoſe the ſides of the hill; and either form a moraſs below, or a weeping rock, by oozing out in numerous places, or many of theſe leſs currents meeting together burſt out in a more copious rill.

The ſummits of mountains are much colder than the plains in their vicinity, owing to ſeveral cauſes; 1. Their being in a manner inſulated or cut off from the common heat of the earth, which is always of 48 degrees, and perpetually counteracts the effects of external cold beneath that degree. 2. From their ſurfaces being larger in proportion to their ſolid contents, and hence their heat more expeditiouſly carried away by the ever-moving atmoſphere. 3. The increaſing rarity of the air as the mountain riſes. All thoſe bodies which conduct electricity well or ill, conduct the matter of heat likewiſe well or ill. See note VII. Atmoſpheric air is a bad conductor of electricity and thence confines it on the body where it is accumulated, but when it is made very rare, as in the exhauſted receiver, the electric aura paſſes away immediately to any diſtance. The ſame circumſtance probably happens in reſpect to heat, which is thus kept by the denſer air on the plains from eſcaping, but is diſſipated on the hills where the air is thinner. 4. As the currents of air riſe up the ſides of mountains they become mechanically rarefied, the preſſure of the incumbent column leſſening as they aſcend. Hence the expanding air abſorbs heat from the mountain as it aſcends, as explained in note VII. 5. There is another, and perhaps more powerful cauſe, I ſuſpect, which may occaſion the great cold on mountains, and in the higher parts of the atmoſphere, and which has not yet been attended to; I mean that the fluid matter of heat may prodably gravitate round the earth, and form an atmoſphere on its ſurface, mixed with the aerial atmoſphere, which may diminiſh or become rarer, as it recedes from the earth's ſurface, in a greater proportion than the air diminiſhes.

[71] 6. The great condenſation of moiſture on the ſummits of hills has another cauſe, which is the daſhing of moving clouds againſt them, in miſty days this is often ſeen to have great effect on plains, where an eminent tree by obſtructing the miſt as it moves along ſhall have a much greater quantity of moiſture drop from its leaves than falls at the ſame time on the ground in its vicinity. Mr. White, in his Hiſtory of Selborne gives an account of a large tree ſo ſituated, from which a ſtream flowed during a moving miſt ſo as to fill the cart-ruts in a lane otherwiſe not very moiſt, and ingeniouſly adds, that trees planted about ponds of ſtagnant water contribute much by theſe means to ſupply the reſervoir. The ſpherules which conſtitute a miſt or cloud are kept from uniting by ſo ſmall a power that a little agitation againſt the leaves of a tree, or the greater attraction of a flat moiſt ſurface, condenſes or precipitates them.

If a leaf has its ſurface moiſtened and particles of water ſeparate from each other as in a miſt be brought near the moiſtened ſurface of a leaf, each particle will be attracted more by that plain ſurface of water on the leaf than it can be by the ſurrounding particles of the miſt, becauſe globules only attract each other in one point, whereas a plain attracts a globule by a greater extent of its ſurface.

The common cold ſprings are thus formed on elevated grounds by the condenſed vapours, and hence are ſtronger when the nights are cold after hot days in ſpring, than even in the wet days of winter. For the warm atmoſphere during the day has diſſolved much more water than it can ſupport in ſolution during the cold of the night, which is thus depoſited in large quantities on the hills, and yet ſo gradually as to ſoak in between the ſtrata of them, rather than to ſlide off over their ſurfaces like ſhowers of rain. The common heat of the internal parts of the earth is aſcertained by ſprings which ariſe from ſtrata of earth too deep to be affected by the heat of ſummer or the froſts of winter. Thoſe in this country are of 48 degrees of heat, thoſe about Philidelphia were ſaid by Dr. Franklin to be 52; whether this variation is to be accounted for by the difference of the ſun's heat on that country, according to the ingenious theory of Mr. Kirwan, or to the vicinity of ſubterranean fires is not yet, I think, decided. There are however ſub⯑terraneous ſtreams of water not exactly produced in this manner, as ſtreams iſſuing from fiſſures in the earth, communicating with the craters of old volcanoes; in the Peak of Derbyſhire are many hollows, called ſwallows, where the land floods ſink into the earth, and come out at ſome miles diſtant, as at Ilam near Aſhborne. See note on Fica, Vol. II.

Other ſtreams of cold water ariſe from beneath the ſnow on the Alps and Andes, and other high mountains, which is perpetualy thawing at its under ſurface by the common heat of the earth, and gives riſe to large rivers. For the origin of warm ſprings ſee note on Fucus, Vol. II.

NOTE XXVII.—SHELL FISH.

THE armour of the Echinus, or Sea-hedge Hog, conſiſts generally of moveable ſpines; (Linnei Syſtem. Nat. Vol. I. p. 1102.) and in that reſpect reſembles the armour of the land animal of the ſame name. The irregular protuberances on other ſea-ſhells, as on ſome ſpecies of the Purpura, and Murex, ſerve them as a fortification againſt the attacks of their enemies.

It is ſaid that this animal foreſees tempeſtuous weather, and ſinking to the bottom of the ſea adheres firmly to ſea-plants, or other bodies by means of a ſubſtance which reſembles the horns of ſnails. Above twelve hundred of theſe fillets have been counted by which this animal fixes itſelf; and when afloat, it contracts theſe fillets between the baſes of its points, the number of which often amounts to two thouſand. Dict. raiſonne. art. Ourſin. de mer.

There is a kind of Nautilus, called by Linneus, Argonauta, whoſe ſhell has but one cell; of this animal Pliny affirms, that having exonerated its ſhell by throwing out the water, it ſwims upon the ſurface, extending a web of wonderful tenuity, and bending back two of its arms and rowing with the reſt, makes a ſail, and at length receiving the water dives again. Plin. IX. 29. Linneus adds to his deſcription of this animal, that like the the Crab Diogenes or Bernhard, it occupies a houſe not its own, as it is not connected to its ſhell, and is therefore foreign to it; who could have given credit to this if it had not been atteſted by ſo many who have with their own eyes ſeen this argonaut in the act of ſailing? Syſt. Nat. p. 1161.

The Nautilus, properly ſo named by Linneus, has a ſhell conſiſting of many cham⯑bers, of which cups are made in the Eaſt with beautiful painting and carving on the mother-pearl. The animal is ſaid to inhabit only the uppermoſt or open chamber, which is larger than the reſt; and that the reſt remain empty except that the pipe, or ſiphun⯑culus, which communicates from one to the other of them is filled with an appendage of the animal like a gut or ſtring. Mr. Hook in his Philos. Exper. p. 306, imagines this to be a dilatable or compreſſible tube, like the air-bladders of fiſh, and that by contracting or permitting it to expand, it renders its ſhell boyant or the contrary. See Note on Ulva, Vol. II.

The Pinna, or Sea-wing, is contained in a two-valve ſhell, weighing ſometimes fifteen pounds, and emits a beard of fine long gloſſy ſilk-like fibres, by which it is ſuſpended to the rocks twenty or thirty feet beneath the ſurface of the ſea. In this ſituation it is ſo ſucceſsfully attacked by the eight-footed Polypus, that the ſpecies perhaps could not exiſt [73] but for the exertions of the Cancer Pinnotheris, who lives in the ſame ſhell as a guard and companion. Amoen. Academ. Vol. II. p. 48. Lin. Syſt. Nat. Vol. I. p. 1159, and p. 1040.

The Pinnotheris, or Pinnophylax, is a ſmall crab naked like Bernard the Hermit, but is furniſhed with good eyes, and lives in the ſame ſhell with the Pinna; when they want food the Pinna opens its ſhell, and ſends its faithful ally to forage; but if the Cancer ſees the Polypus, he returns ſuddenly to the arms of his blind hoſteſſs, who by cloſing the ſhell avoids the fury of her enemy; otherwiſe, when it has procured a booty, it brings it to the opening of the ſhell, where it is admitted, and they divide the prey. This was obſerved by Haſlequiſt in his voyage to Paleſtine.

The Byſſus of the antients, according to Ariſtotle, was the beard of the Pinna above mentioned, but ſeems to have been uſed by other writers indiſcriminately for any ſpun material, which was eſteemed finer or more valuable than wool. Reaumur ſays the threads of this Byſſus are not leſs fine or leſs beautiful than the ſilk, as it is ſpun by the ſilk-worm; the Pinna on the coaſts of Italy and Provence (where it is fiſhed up by iron⯑hooks fixed on long poles) is called the ſilk-worm of the ſea. The ſtockings and gloves manufactured from it, are of exquiſite fineneſs, but too warm for common wear, and are thence eſteemed uſeful in rhumatiſm and gout. Dict. Raiſonné art. Pinne-marine. The warmth of the Byſſus, like that of ſilk, is probably owing to their being bad conductors of heat, as well as of electricity. When theſe fibres are broken by violence, this animal as well as the muſcle has the power to reproduce them like the common ſpiders, as was obſerved by M. Adanſon. As raw ſilk, and raw cobwebs, when ſwallowed, are liable to produce great ſickneſs (as I am informed) it is probable the part of muſcles, which ſometimes diſagrees with the people who eat them, may be this ſilky web, by which they attach themſelves to ſtones. The large kind of Pinna contains ſome mother-pearl of a reddiſh tinge, according to M. d'Argenville. The ſubſtance ſold under the name of Indian weed, and uſed at the bottom of fiſh-lines, is probably a production of this kind, which however is ſcarcely to be diſtinguiſhed by the eye from the tendons of a rat's tail, after they have been ſeparated by putrefaction in water, and well cleaned and rubbed; a production, which I was once ſhewn as a great curioſity; it had the upper⯑moſt bone of the tail adhering to it, and was ſaid to have been uſed as an ornament in a lady's hair.

NOTE XXVIII.—STURGEON.

THE Sturgeon, Acipenſer, Strurio. Lin. Syſt. Nat. Vol. I. p. 403. is a fiſh of great cu⯑rioſity as well as of great importance; his mouth is placed under the head, without teeth, like the opening of a purſe, which he has the power to puſh ſuddenly out or retract. Before this mouth under the beak or noſe hang four tendrils ſome inches long, and which ſo reſemble earth-worms that at firſt ſight they may be miſtaken for them. This clumſy toothleſs fiſh is ſuppoſed by this contrivance to keep himſelf in good condition, the ſolidity of his fleſh evidently ſhewing him to be a fiſh of prey. He is ſaid to hide his large body amongſt the weeds near the ſea-coaſt, or at the mouths of large rivers, only expoſing his cirrhi or tendrils, which ſmall fiſh or ſea-inſects miſtaking for real worms approach for plunder, and are ſucked into the jaws of their enemy. He has been ſup⯑poſed by ſome to root into the ſoil at the bottom of the ſea or rivers; but the cirrhi, or tendrills abovementioned, which hang from his ſnout over his mouth, muſt themſelves be very inconvenient for this purpoſe, and as it has no jaws it evidently lives by ſuction, and during its reſidence in the ſea a quantity of ſea-inſects are found in its ſtomach.

The fleſh was ſo valued in the time of the Emperor Severus, that it was brought to table by ſervants with coronets on their heads, and preceded by muſic, which might give riſe to its being in our country preſented by the Lord Mayor to the King. At preſent it is caught in the Danube, and the Walga, the Don, and other large rivers for various pur⯑poſes. The ſkin makes the beſt covering for carriages; iſinglaſs is prepared from parts of the ſkin; cavear from the ſpawn; and the fleſh is pickled or ſalted, and ſent all over Europe.

NOTE XXIX.—OIL ON WATER.

THERE is reaſon to believe that when oil is poured upon water, the two ſurfaces do not touch each other, but that the oil is ſuſpended over the water by their mutual repulſion. This ſeems to be rendered probable by the following experiment: if one drop of oil be droped on a baſon of water, it will immediately diffuſe itſelf over the whole, for there being no friction between the two ſurfaces, there is nothing to prevent its ſpreading itſelf by the gravity of the upper part of it, except its own tenacity, into a pellicle [75] of the greateſt tenuity. But if a ſecond drop of oil be put upon the former, it does not ſpread itſelf, but remains in the form of a drop, as the other already occupied the whole ſurface of the baſon, and there is friction in oil paſſing over oil, though none in oil paſſing over water.

Hence when oil is diffuſed on the ſurface of water gentle breezes have no influence in raiſing waves upon it; for a ſmall quantity of oil will cover a very great ſurface of water, (I ſuppoſe a ſpoonful will diffuſe itſelf over ſome acres) and the wind blowing upon this carries it gradually forwards; and there being no friction between the two ſurfaces the water is not affected. On which account oil has no effect in ſtilling the agitation of the water after the wind ceaſes, as was found by the experiments of Dr. Franklin.

This circumſtance lately brought into notice by Dr. Franklin had been mentioned by Pliny, and is ſaid to be in uſe by the divers for pearls, who in windy weather taken down with them a little oil in their mouths, which they occaſionally give out when the in⯑equality of the ſupernatant waves prevents them from ſeeing ſufficiently diſtinctly for their purpoſe.

The wonderful tenuity with which oil can be ſpread upon water is evinced by a few drops projected from a bridge, where the eye is properly placed over it, paſſing through all the priſmatic colours as it diffuſes itſelf. And alſo from another curious ex⯑periment of Dr. Franklin's: he cut a piece of cork to about the ſize of a letter-wafer, leaving a point ſtanding off like a tangent at one edge of the circle. This piece of cork was then dipped in oil and thrown into a large pond of water, and as the oil flowed off at the point, the cork-wafer continued to revolve in a contrary direction for ſeveral minutes. The oil flowing off all that time at the pointed tangent in coloured ſtreams. In a ſmall pond of water this experiment does not ſo well ſucceed, as the circulation of the cork ſtops as ſoon as the water becomes covered with the pellicle of oil. See Additional Note, No. XIII. and Note on Fucus, Vol. II.

The eaſe with which oil and water ſlide over each other is agreeably ſeen if a phial be about half filled with equal parts of oil and water, and made to oſcillate ſuſpended by a ſtring, the upper ſurface of the oil and the lower one of the water will always keep ſmooth; but the agitation of the ſurfaces where the oil and water meet, is curious; for their ſpecific gravities being not very different, and their friction on each other nothing, the higheſt ſide of the water, as the phial deſcends in its oſcillation, having acquired a greater momentum than the loweſt ſide (from its having deſcended further) would riſe the higheſt on the aſcending ſide of the oſcillation, and thence puſhes the then uppermoſt part of the water amongſt the oil.

NOTE XXX.—SHIP-WORM.

THE Teredo, or ſhip-worm, has two calcareous jaws, hemiſpherical, flat before, and angular behind. The ſhell is taper, winding, penetrating ſhips and ſubmarine wood, and was brought from India into Europe, Linnei Syſtem. Nat. p. 1267. The Tarieres, or ſea-worms, attack and erode ſhips with ſuch fury, and in ſuch num⯑bers, as often greatly to endanger them. It is ſaid that our veſſels have not known this new enemy above fifty years, that they were brought from the ſea about the Antilles to our parts of the ocean, where they have increaſed prodigiouſly. They bore their paſſage in the direction of the fibres of the wood, which is their nouriſhment, and cannot return or paſs obliquely, and thence when they come to a knot in the wood, or when two of them meet together with their ſtony mouths, they periſh for want of food.

In the years 1731 and 1732 the United Provinces were under a dreadful alarm concerning theſe inſects, which had made great depredation on the piles which ſupport the banks of Zeland, but it was happily diſcovered a few years afterwards that theſe inſects had totally abandoned that iſland, (Dict. Raiſonné, art. Vers Rongeurs,) which might have been occaſioned by their not being able to live in that latitude when the winter was rather ſeverer than uſual.

NOTE XXXI.—MAELSTROM.

ON the coaſt of Norway there is an extenſive vortex, or eddy, which lies between the iſlands of Moſkoe and Moſkenas, and is called Moſkoeſtrom, or Maelſtrom; it occupies ſome leagues in circumference, and is ſaid to be very dangerous and often deſtructive to veſſels navigating theſe ſeas. It is not eaſy to underſtand the exiſtence of a conſtant deſcending ſtream without ſuppoſing it muſt paſs through a ſubterranean cavity to ſome other part of the earth or ocean which may lie beneath its level; as the Mediterranean ſeems to lie beneath the level of the Atlantic ocean, which therefore [77] conſtantly flows into it through the Straits; and the waters of the Gulph of Mexico lie much above the level of the ſea about the Floridas and further northward, which gives riſe to the Gulph-ſtream, as deſcribed in note on Caſſia in Vol. II.

The Maelſtrom is ſaid to be ſtill twice in about twenty-four hours when the tide is up, and moſt violent at the oppoſite times of the day. This is not difficult to account for, ſince when ſo much water is brought over the ſubterraneous paſſage, if ſuch exiſts, as compleatly to fill it and ſtand many feet above it, leſs diſturbance muſt appear on the ſurface. The Maelſtrom is deſcribed in the Memoires of the Swediſh Academy of Sciences, and Pontoppiden's Hiſt. of Norway, and in Univerſal Muſeum for 1763, p. 131.

The reaſon why eddies of water become hollow in the middle is becauſe the water immediately over the centre of the well, or cavity, falls faſter, having leſs friction to oppoſe its deſcent, than the water over the circumference or edges of the well. The circular motion or gyration of eddies depends on the obliquity of the courſe of the ſtream, or to the friction or oppoſition to it being greater on one ſide of the well than the other; I have obſerved in water paſſing through a hole in the bottom of a trough, which was always kept full, the gyration of the ſtream might be turned either way by increaſing the oppoſition of one ſide of the eddy with ones finger, or by turning the ſpout, through which the water was introduced, a little more obliquely to the hole on one ſide or on the other. Lighter bodies are liable to be retained long in eddies of water, while thoſe rather heavier than water are ſoon thrown out beyond the circum⯑ference by their acquired momentum becoming greater than that of the water. Thus if equal portions of oil and water be put into a phial, and by means of a ſtring be whirled in a circle round the hand, the water will always keep at the greater diſtance from the centre, whence in the eddies formed in rivers during a flood a perſon who endeavours to keep above water or to ſwim is liable to be detained in them, but on ſuffering himſelf to ſink or dive he is ſaid readily to eſcape. This circulation of water in deſcending through a hole in a veſſel Dr. Franklin has ingeniouſly applied to the explanation of hurricanes or eddies of air.

NOTE XXXII.—GLACIERS.

THE common heat of the interior parts of the earth being always 48 degrees, both in winter and ſummer, the ſnow which lies in contact with it is always in a thawing ſtate; Hence in ice-houſes the external parts of the collection of ice is perpetually thawing and thus preſerves the internal part of it; ſo that it is neceſſary to lay up many tons for the preſervation of one ton. Hence in Italy conſiderable rivers have their ſource from beneath the eternal glaciers, or mountains of ſnow and ice.

In our country when the air in the courſe of a froſt continues a day or two at very near 32 degrees, the common heat of the earth thaws the ice on its ſurface, while the thermometer remains at the freezing point. This circumſtance is often obſervable in the rimy mornings of ſpring; the thermometer ſhall continue at the freezing point, yet all the rime will vaniſh, except that which happens to lie on a bridge, a board, or on a cake of cow-dung, which being thus as it were inſulated or cut off from ſo free a com⯑munication with the common heat of the earth by means of the air under the bridge, or wood, or dung, which are bad conductors of heat, continues ſome time longer unthawed. Hence when the ground is covered thick with ſnow, though the froſt continues, and the ſun does not ſhine, yet the ſnow is obſerved to decreaſe very ſenſibly. For the common heat of the earth melts the under ſurface of it, and the upper one evaporates by its ſolution in the air. The great evaporation of ice was obſerved by Mr. Boyle, which experiment I repeated ſome time ago. Having ſuſpended a piece of ice by a wire and weighed it with care without touching it with my hand, I hung it out the whole of a clear froſty night, and ſound in the morning it had loſt nearly a fifth of its weight. Mr. N. Wallerius has ſince obſerved that ice at the time of its congelation evaporates faſter than water in its fluid form; which may be accounted for from the heat given out at the inſtant of freezing; (Sauſſure's Eſſais ſur Hygromet. p. 249.) but this effect is only momentary.

Thus the vegetables that are covered with ſnow are ſeldom injured; ſince, as they lie between the thawing ſnow, which has 32 degrees of heat, and the covered earth which has 48, they are preſerved in a degree of heat between theſe; viz. in 40 degrees of heat. Whence the moſs on which the rein-deer feed in the northern latitudes vegetates beneath the ſnow; (See note on Muſchus, Vol. II.) and hence many Lapland and Alpine plants periſhed through cold in the botanic garden at Upſal, for in their native ſituations, though the cold is much more intenſe, yet at its very commencement they are covered deep with ſnow, which remains till late in the ſpring. For this fact ſee Amaenit. Academ. Vol. I. No. 48. In our climate ſuch plants do well covered with dried fern, under which they will grow, and even flower, till the ſevere vernal froſts ceaſe. For the increaſe of glaciers ſee Note on Canto I. l. 529.

NOTE XXXIII.—WINDS.

THE theory of the winds is yet very imperfect, in part perhaps owing to the want of obſervations ſufficiently numerous of the exact times and places where they begin and ceaſe to blow, but chiefly to our yet imperfect knowledge of the means by which great regions of air are either ſuddenly produced or ſuddenly deſtroyed.

The air is perpetually ſubject to increaſe or diminution from its combination with other bodies, or its evolution from them. The vital part of the air, called oxygene, is continually produced in this climate from the perſpiration of vegetables in the ſunſhine, and probably from the action of light on clouds or on water in the tropical climates, where the ſun has greater power, and may exert ſome yet umknown laws of luminous combination. Another part of the atmoſphere, which is called azote, is perpetually ſet at liberty from animal and vegetable bodies by putrefaction or combuſtion, from many ſprings of water, from volatile alcali, and probably from fixed alcali, of which there is an exhauſtleſs ſource in the water of the ocean. Both theſe component parts of the air are perpetually again diminiſhed by their contact with the ſoil, which covers the ſurface of the earth, producing nitre. The oxygene is diminiſhed in the production of all acids, of which the carbonic and muriatic exiſt in great abundance. The azote is diminiſhed in the growth of animal bodies, of which it conſtitutes an important part, and in its combinations with many other natural productions.

They are both probably diminiſhed in immenſe quantities by uniting with the inflammable air, which ariſes from the mud of rivers and lakes at ſome ſeaſons, when the atmoſphere is light: the oxgene of the air producing water, and the azote producing volatile alcali by their combinations with this inflammable air. At other ſeaſons of the year theſe principles may again change their combinations, and the atmoſpheric air be reproduced.

Mr. Lavoiſier found that one pound of charcoal in burning conſumed two pounds nine ounces of vital air, or oxygene. The conſumption of vital air in the proceſs of making red lead may readily be reduced to calculation; a ſmall barrel contains about twelve hundred weight of this commodity, 1200 pounds of lead by calcination abſorb about 144 pounds of vital air; now as a cubic foot of water weighs 1000 averdupois ounces, and as vital air is above 800 times lighter than water, it follows that every barrel of red lead contains nearly 2000 cubic feet of vital air. If this can be performed in miniature in a ſmall oven, what may not be done in the immenſe elaboratories of nature!

Theſe great elaboratories of nature include almoſt all her foſſil as well as her animal and vegetable productions. Dr. Prieſtley obtained air of greater or leſs purity, both [80] vital and azotic, from almoſt all the foſſil ſubſtances he ſubjected to experiment. Four ounce-weight of lava from Iceland heated in an earthen retort yielded twenty ounce-meaſures of air.

In this account the fixed air was previouſly extracted from the limeſtones by acids, and the heat applied was much leſs than was neceſſary to extract all the air from the bodies employed. Add to this the known quantities of air which are combined with the calciform ores, as the ochres of iron, manganeſe, calamy, grey ore of lead, and ſome idea may be formed of the great production of air in volcanic eruptions, as mentioned in note on Chunda, Vol. II. and of the perpetual abſorptions and evolutions of whole oceans of air from every part of the earth.

But there would ſeem to be an officina aeris, a ſhop where air is both manufactured and deſtroyed in the greateſt abundance within the polar circles, as will hereafter be ſpoken of. Can this be effected by ſome yet unknown law of the congelation of aqueous or ſaline fluids, which may ſet at liberty their combined heat, and convert a part both of the acid and alcali of ſea-water into their component airs? Or on the contrary can the electricity of the northern lights convert inflammable air and oxygene into water, whilſt the great degree of cold at the poles unites the azote with ſome other baſe? Another officina aeris, or manufacture of air, would ſeem to exiſt within the tropics or at the line, though in a much leſs quantity than at the poles, owing perhaps to the action of the ſun's light on the moiſture ſuſpended in the air, as will alſo be ſpoken of hereafter; but in all other parts of the earth theſe abſorptions and evolutions of air in a greater or leſs degree are perpetually going on in inconceivable abundance; increaſed probably, and diminiſhed at different ſeaſons of the year by the approach or retroceſſion of the ſun's light; future diſcoveries muſt elucidate this part of the ſubject. To this ſhould be added [81] that as heat and electricity, and perhaps magnetiſm, are known to diſplace air, that it is not impoſſible but that the increaſed or diminiſhed quantities of theſe fluids diffuſed in the atmoſphere may increaſe its weight a well as its bulk; ſince their ſpecific attractions or affinities to matter are very ſtrong, they probably alſo poſſeſs general gravitation to the earth; a ſubject which wants further inveſtigation. See Note XXVI.

NOTE XXXIV.—VEGETABLE PERSPIRATION.

WHEN points or hairs are put into ſpring-water, as in the experiments of Sir B. Thompſon, (Philoſ. Tranſ. Vol. LXXVII.) and expoſed to the light of the ſun, much air, which looſely adhered to the water, riſes in bubbles, as explained in note on Fucus, Vol. II. A ſtill greater quantity of air, and of a purer kind, is emitted by Dr. Prieſtley's green matter, and by vegetable leaves growing in water in the ſun-ſhine, according to Mr. Ingenhouze's experiments; both which I ſuſpect to be owing to a decompoſition of the water perſpired by the plant, for the edge of a capillary tube of great tenuity may be conſidered as a circle of points, and as the oxygene, or principle of vital air, may be ex⯑panded into a gas by the ſun's light; the hydrogene or inflammable air may be detained in the pores of the vegetable.

Hence plants growing in the ſhade are white, and become green by being expoſed to the ſun's light; for their natural colour being blue, the addition of hydrogene adds yellow to this blue, and tans them green. I ſuppoſe a ſimilar circumſtance takes place in animal bodies; their perſpirable matter as it eſcapes in the ſun-ſhine becomes decom⯑poſed by the edges of their pores as in vegetables, though in leſs quantity, as their per⯑ſpiration is leſs, and by the hydrogene being retained the ſkin becomes tanned yellow. In proof of this it muſt be obſerved that both vegetable and animal ſubſtances become bleached white by the ſun-beams when they are dead, as cabbage-ſtalks, bones, ivory, tallow, bees-wax, linen and cotton cloth; and hence I ſuppoſe the copper-coloured natives of ſunny countries might become etiolated or blanched by being kept from their infancy in the dark, or removed for a few generations to more northerly climates.

It is probable that on a ſunny morning much pure air becomes ſeparated from the dew by means of the points of vegetables on which it adheres, and much inflammable air imbibed by the vegetable, or combined with it; and by the ſun's light thus decom⯑poſing water the effects of it in bleaching linen ſeems to depend (as deſcribed in Note X.): the water is decompoſed by the light at the ends or points of the cotton or thread, and the vital air unites with the phlogiſtic or colouring matters of the cloth, and produces a new acid, which is either itſelf colourleſs or waſhes out, at the ſame time the inflammable part of the water eſcapes. Hence there ſeems a reaſon why cotton bleaches ſo much ſooner than linen, viz. becauſe its fibres are three or four times ſhorter, and therefore protrude ſo many more points, which ſeem to facilitate the liberation of the vital air from the inflammable part of the water.

Bee's wax becomes bleached by expoſure to the ſun and dews in a ſimilar manner as metals become calcined or ruſty, viz. by the water on their ſurface being decompoſed; and hence the inflammable material which cauſed the colour becomes united with vital air forming a new acid, and is waſhed away.

[95] Oil cloſe ſtopped in a phial not full, and expoſed long to the ſun's light, becomes bleached, as I ſuppoſe, by the decompoſition of the water it contains; the inflammable air riſing above the ſurface, and the vital air uniting with the colouring matter of the oil. For it is remarkable, that by ſhutting up a phial of bleached oil in a dark drawer, it in a little time becomes coloured again.

The following experiment ſhews the power of light in ſeparating vital air from another baſis, viz. from azote. Mr. Scheel inverted a glaſs veſſel filled with colourleſs nitrous acid into another glaſs containing the ſame acid, and on expoſing them to the ſun's light, the inverted glaſs became partly filled with pure air, and the acid at the ſame time became coloured. Scheel in Crell's Annal. 1786. But if the veſſel of colourleſs nitrous acid be quite full and ſtopped, ſo that no ſpace is left for the air produced to ex⯑pand itſelf into, no change of colour takes place. Prieſtley's Exp. VI. p. 344. See Keir's very excellent Chemical Dictionary, p. 99. new edition.

A ſun-flower three feet and half high according to the experiment of Dr. Hales, per⯑ſpired two pints in one day (Vegetable Statics.) which is many times as much in pro⯑portion to its ſurface, as is perſpired from the ſurface and lungs of animal bodies; it follows that the vital air liberated from the ſurfaces of plants by the ſunſhine muſt much exceed the quantity of it abſorbed by their reſpiration, and that hence they improve the air in which they live during the light part of the day, and thus blanched vegetables will ſooner become tanned into green by the ſun's light, than etiolated animal bodies will be⯑come tanned yellow by the ſame means.

It is hence evident, that the curious diſcovery of Dr. Prieſtley, that his green vegetable matter and other aquatic plants gave out vital air when the ſun ſhone upon them, and the leaves of other plants did the ſame when immerſed in water, as obſerved by Mr. Ingenhouze, refer to the perſpiration of vegetables not to their reſpiration. Becauſe Dr. Prieſtley obſerved the pure air to come from both ſides of the leaves and even from the ſtalks of a water-flag, whereas one ſide of the leaf only ſerves the of the office of lungs, and certainly not the ſtalks. Exper. on Air, Vol. III. And thus in reſpect to the circum⯑ſtance in which plants and animals ſeemed the furthereſt removed from each other, I mean in their ſuppoſed mode of reſpiration, by which one was believed to purify the air which the other had injured, they ſeem to differ only in degree, and the analogy between them remains unbroken.

Plants are ſaid by many writers to grow much faſter in the night than in the day; as is particularly obſervable in ſeedlings at their riſing out of the ground. This probably is a conſequence of their ſleep rather than of the abſence of light; and in this I ſuppoſe they alſo reſemble animal bodies.

NOTE XXXV.—VEGETABLE PLACENTATION.

AS buds are the viviparous offspring of vegetables, it becomes neceſſary that they ſhould be furniſhed with placental veſſels for their nouriſhment, till they acquire lungs or leaves for the purpoſe of elaborating the common juices of the earth into nutri⯑ment. Theſe veſſels exiſt in bulbs and in ſeeds, and ſupply the young plant with a ſweet juice till it acquires leaves, as is ſeen in converting barley into malt, and appears from the ſweet taſte of onions and potatoes, when they begin to grow.

The placental veſſels belonging to the buds of trees are placed about the roots of moſt, as the vine; ſo many roots are furniſhed with ſweet or mealy matter as fern-root, bryony, carrot, turnip, potatoe, or in the alburnum or ſap-wood as in thoſe trees which produce manna, which is depoſited about the month of Auguſt, or in the joints of ſugar cane, and graſſes; early in the ſpring the abſorbent mouths of theſe veſſels drink up moiſture from the earth, with a ſaccharine matter lodged for that purpoſe during the preceding autumn, and puſh this nutritive fluid up the veſſels of the alburnum to every individual bud, as is evinced by the experiments of Dr. Hales, and of Mr. Walker in the Edinburgh Philoſophical Tranſact. The former obſerved that the ſap from the ſtump of a vine, which he had cut off in the beginning of April, aroſe twenty-one feet high in tubes affixed to it for that purpoſe, but in a few weeks it ceaſed to bleed at all, and Dr. Walker marked the progreſs of the aſcending ſap, and found likewiſe that as ſoon as the leaves became expanded the ſap ceaſed to riſe; the aſcending juice of ſome trees is ſo copious and ſo ſweet during the ſap-ſeaſon that it is uſed to make wine, as the birch, betula, and ſycamore, acer pſeudo-platinus, and particularly the palm.

During this aſcent of the ſap-juice each individual leaf-bud expands its new leaves, and ſhoots down new roots, covering by their intertexture the old bark with a new one; and as ſoon as theſe new roots (or bark) are capable of abſorbing ſufficient juices from the earth for the ſupport of each bud, and the new leaves are capable of performing their office of expoſing theſe juices to the influence of the air; the placental veſſels ceaſe to act, coaleſce, and are tranſformed from ſap-wood, or alburnum, into inert wood; ſerving only for the ſupport of the new tree, which grows over them.

Thus from the pith of the new bud of the horſe-cheſnut five veſſels paſs out through the circle of the placental veſſels above deſcribed, and carry with them a minuter circle of thoſe veſſels; theſe five bundles of veſſels unite after their exit, and form the foot⯑ſtalk or petiole of the new five-fingered leaf, to be ſpoken of hereafter. This ſtructure is well ſeen by cutting off a leaf of the horſe-cheſnut (Aeſculus Hippocaſtanum) in September before it falls, as the buds of this tree are ſo large that the flower may be ſeen in them with the naked eye.

[97] After a time, perhaps about midſummer, another bundle of veſſels paſſes from the pith through the alburnum or ſap-veſſels in the boſom of each leaf, and unites by the new bark with the leaf, which becomes either a flower-bud or a leaf-bud to be expanded in the enſuing ſpring, for which purpoſe an apparatus of placental veſſels are produced with proper nutriment during the progreſs of the ſummer and autumn, and thus the vegetable becomes annually increaſed, ten thouſand buds often exiſting on one tree, according to the eſtimate of Linneus. Phil. Bot.

The vaſcular connection of vegetable buds with the leaves in whoſe boſoms they are formed is confirmed by the following experiment, (Oct. 20, 1781.) On the extremity of a young bud of the Mimoſa (ſenſitive plant) a ſmall drop of acid of vitriol was put by means of a pen, and, after a few ſeconds, the leaf in whoſe axilla it dwelt cloſed and opened no more, though the drop of vitriolic acid was ſo ſmall as apparently only to injure the ſummit of the bud. Does not this ſeem to ſhew that the leaf and its bud have connecting veſſels though they ariſe at different times and from different parts of the medulla or pith? And, as it exiſts previouſly to it, that the leaf is the parent of the bud?

This placentation of vegetable buds is clearly evinced from the ſweetneſs of the riſing ſap, and from its ceaſing to riſe as ſoon as the leaves are expanded, and thus compleats the analogy between buds and bulbs. Nor need we wonder at the length of the umbilical cords of buds ſince that muſt correſpond with their ſituation on the tree, in the ſame manner as their lymphatics and arteries are proportionally elongated.

It does not appear probable that any umbilical artery attends theſe placental ab⯑ſorbents, ſince, as there ſeems to be no ſyſtem of veins in vegetables to bring back the blood from the extremities of their arteries, (except their pulmonary veins,) there could not be any vegetable fluids to be returned to their placenta, which in vege⯑tables ſeems to be ſimply an organ for nutrition, whereas the placenta of the animal foetus ſeems likewiſe to ſerve as a reſpiratory organ like the gills of fiſhes.

NOTE XXXVI.—VEGETABLE CIRCULATION.

THE individuality of vegetable buds was ſpoken of before, and is confirmed by the method of raiſing all kinds of trees by Mr. Barnes. (Method of propagating Fruit Trees. 1759. Lond. Baldwin.) He cut a branch into as many pieces as there were buds or leaves upon it, and wiping the two wounded ends dry he quickly applied to each a cement, previouſly warmed a little, which conſiſted principally of pitch, and planted them in the earth. The uſe of this cement I ſuppoſe to conſiſt in its preventing the bud from bleeding to death, though the author aſcribes it to its antiſceptic quality.

Theſe buds of plants, which are thus each an individual vegetable, in many circum⯑ſtances reſemble individual animals, but as animal bodies are detached from the earth, and move from place to place in ſearch of food, and take that food at conſiderable intervals of time, and prepare it for their nouriſhment within their own bodies after it is taken, it is evident they muſt require many organs and powers which are not neceſſary to a ſtationary bud. As vegetables are immoveably fixed to the ſoil from whence they draw their mouriſhment ready prepared, and this uniformly not at returning intervals, it follows that in examining their anatome we are not to look for muſcles of locomotion, as arms and legs; nor for organs to receive and prepare their nouriſhment, as a ſtomach and bowels; nor for a reſervoir for it after it is prepared, as a general ſyſtem of veins, which in locomotive animals contains and returns the ſuperfluous blood which is left after the various organs of ſecretion have been ſupplied, by which con⯑trivance they are enabled to live a long time without new ſupplies of food.

The parts which we may expect to find in the anatome of vegetables correſpondent to thoſe in the animal economy are, 1. A ſyſtem of abſorbent veſſels to imbibe the moiſture of the earth ſimilar to the lacteal veſſels, as in the roots of plants; and another ſyſtem of abſorbents ſimilar to the lymphatics of animal bodies, opening its mouths on the internal cells and external ſurfaces of vegetables; and a third ſyſtem of ab⯑ſorbent veſſels correſpondent with thoſe of the placentation of the animal foetus. 2. A pulmonary ſyſtem correſpondent to the lungs or gills of quadrupeds and fiſh, by which the fluid abſorbed by the lacteals and lymphatics may be expoſed to the in⯑fluence of the air, this is done by the green leaves of plants, thoſe in the air re⯑ſembling lungs, and thoſe in the water reſembling gills; and by the petals of flowers. 3. Arterial ſyſtems to convey the fluid thus elaborated to the various glands of the vegetable for the purpoſes of its growth, nutrition, and various ſecretions. 4. The various glands which ſeparate from the vegetable blood the honey, wax, gum, reſin, ſtarch, ſugar, eſſential oil, &c. 5. The organs adapted for their propagation or re⯑production. 6. Muſcles to perform ſeveral motions of their parts.

[99] I. The exiſtence of that branch of the abſorbent veſſels of vegetables which reſembles the lacteals of animal bodies, and imbibes their nutriment from the moiſt earth, is evinced by their growth ſo long as moiſture is applied to their roots, and their quickly withering when it is withdrawn.

Beſides theſe abſorbents in the roots of plants there are others which open their mouths on the external ſurfaces of the bark and leaves, and on the internal ſurfaces of all the cells, and between the bark and the alburnum or ſap-wood; the exiſtence of theſe is ſhewn, becauſe a leaf plucked off and laid with its under ſide on water will not wither ſo ſoon as if left in the dry air,—the ſame if the bark alone of a branch which is ſeparated from a tree be kept moiſt with water,—and laſtly, by moiſtening the alburnum or ſap-wood alone of a branch detached from a tree it will not ſo ſoon wither as if left in the dry air. By the following experiment theſe veſſels were agreeably viſible by a common magnifying glaſs, I placed in the ſummer of 1781 the footſtalks of ſome large fig-leaves about an inch deep in a decoction of madder, (rubia tinctorum,) and others in a decoction of logwood, (haematoxylum campechenſe,) along with ſome ſprigs cut off from a plant of picris, theſe plants were choſen becauſe their blood is white, after ſome hours, and on the next day, on taking out either of theſe and cutting off from its bottom about a quarter of an inch of the ſtalk an internal circle of red points appeared, which were the ends of abſorbent veſſels coloured red with the decoction, while an external ring of arteries was ſeen to bleed out haſtily a milky juice, and at once evinced both the abſorbent and arterial ſyſtem. Theſe abſorbent veſſels have been called by Grew, and Malphigi, and ſome other philoſophers, bronchi, and erroneouſly ſuppoſed to be air-veſſels. It is probable that theſe veſſels, when cut through, may effuſe their fluids, and receive air, their ſides being too ſtiff to collapſe; ſince dry wood emits air⯑bubles in the exhauſted receiver in the ſame manner as moiſt wood.

The ſtructure of theſe vegetable abſorbents conſiſts of a ſpiral line, and not of a veſſel interrupted with valves like the animal lymphatics, ſince on breaking almoſt any tender leaf and drawing out ſome of the fibres which adhere longeſt this ſpiral ſtructure becomes viſible even to the naked eye, and diſtinctly ſo by the uſe of a common lens. See Grew, Plate 51.

In ſuch a ſtructure it is eaſy to conceive how a vermicular or periſtaltic motion of the veſſel beginning at the loweſt part of it, each ſpiral ring ſucceſſively contracting itſelf till it fills up the tube, muſt forcibly puſh forwards its contents, as from the roots of vines in the bleeding ſeaſon; and if this vermicular motion ſhould begin at the upper end of the veſſel it is as eaſy to ſee how it muſt carry its contained fluid in a contrary direction. The retrograde motion of the vegetable abſorbent veſſels is ſhewn by cutting a forked branch from a tree, and immerſing a part of one of the forks in water, which will for many days prevent the other from withering; or it is ſhewn by planting a willow branch with the wrong end upwards. This ſtructure in ſome degree obtains in the eſophagus or throat of cows, who by ſimilar means convey their food firſt downwards [100] and afterward upwards by a retrograde motion of the annular muſcles or cartilages for the purpoſe of a ſecond maſtication of it.

II. The fluids thus drank up by the vegetable abſorbent veſſels from the earth, or from the atmoſphere, or from their own cells and interſtices, are carried to the foot-ſtalk of every leaf, where the abſorbents belonging to each leaf unite into branches, forming ſo many pulmonary arteries, and are thence diſperſed to the extremities of the leaf, as may be ſeen in cutting away ſlice after ſlice the footſtalk of a horſe-cheſnut in September before the leaf falls. There is then a compleat circulation in the leaf, a pulmonary vein receiving the blood from the extremities of each artery on the upper ſide of the leaf, and joining again in the footſtalk of the leaf theſe veins produce ſo many arteries, or aortas, which diſperſe the new blood over the new bark, elongating its veſſels, or pro⯑ducing its ſecretions; but as a reſervoir of blood could not be wanted by a vegetable bud which takes in its nutriment at all times, I imagine there is no venous ſyſtem, no veins properly ſo called, which receive the blood which was to ſpare, and return it into the pulmonary or arterial ſyſtem.

The want of a ſyſtem of veins was countenanced by the following experiment; I cut off ſeveral ſtems of tall ſpurge, (Euphorbia helioſcopia) in autumn, about the centre of the plant, and obſerved tenfold the quantity of milky juice ooze from the upper than from the lower extremity, which could hardly have happened if there had been a venous ſyſtem of veſſels to return the blood from the roots to the leaves.

Thus the vegetable circulation, complete in the lungs, but probably in the other part of the ſyſtem deficient in reſpect to a ſyſtem of returning veins, is carried forwards without a heart, like the circulation through the livers of animals where the blood brought from the inteſtines and meſentery by one vein is diſperſed through the liver by the vena portarum, which aſſumes the office of an artery. See Note XXXVII.

At the ſame time ſo minute are the veſſels in the intertexture of the barks of plants, which belong to each individual bud, that a general circulation may poſſibly exiſt, though we have not yet been able to diſcover the venous part of it.

There is however another part of the circulation of vegetable juices viſible to the naked eye, and that is in the corol or petals of flowers, in which a part of the blood of the plant is expoſed to the influence of the air and light in the ſame manner as in the foliage, as will be mentioned more at large in Notes XXXVII and XXXIX.

Theſe circulations of their reſpective fluids ſeem to be carried on in the veſſels of plants preciſely as in animal bodies by their irritability to the ſtimulus of their adapted fluids, and not by any mechanical or chemical attraction, for their abſorbent veſſels propel the juice upwards, which they drink up from the earth, with great violence; I ſuppoſe with much greater than is exerted by the lacteals of animals, probably owing to the greater minuteneſs of theſe veſſels in vegetables and the greater rigidity of their coats. Dr. Hales in the ſpring ſeaſon cut off a vine near the ground, and by fixing tubes on the remaining ſtump of it, found the ſap to riſe twenty-one feet in the tube by the propulſive [101] power of theſe abſorbents of the roots of it. Veget. Stat. p. 102. Such a power can not be produced by capillary attraction, as that could only raiſe a fluid nearly to the upper edge of the attracting cylinder, but not enable it to flow over that edge, and much leſs to riſe 21 feet above it. What then can this power be owing to? Doubtleſs to the living activity of the abſorbent veſſels, and to their increaſed vivacity from the influence of the warmth of the ſpring ſucceeding the winter's cold, and their thence greater ſuſceptibility to irritation from the juices which they abſorb, reſembling in all circumſtances the action of the living veſſels of animals.

NOTE XXXVII.—VEGETABLE RESPIRATION.

I. THERE have been various opinions concerning the uſe of the leaves of plants in the vegetable oeconomy. Some have contended that they are perſpiratory organs; this does not ſeem probable from an experiment of Dr. Hales, Veg. Stat. p. 30. He found by cutting off branches of trees with apples on them, and taking off the leaves, that an apple exhaled about as much as two leaves, the ſurfaces of which were nearly equal to the apple; whence it would appear that apples have as good a claim to be termed per⯑ſpiratory organs as leaves. Others have believed them excretory organs of excremen⯑tious juices; but as the vapour exhaled from vegetables has no taſte, this idea is no more probable than the other; add to this that in moiſt weather, they do not appear to per⯑ſpire or exhale at all.

The internal ſurface of the lungs or air-veſſels in men, are ſaid to be equal to the ex⯑ternal ſurface of the whole body, or about fifteen ſquare feet; on this ſurface the blood is expoſed to the influence of the reſpired air through the medium however of a thin pel⯑licle; by this expoſure to the air it has its colour changed from deep red to bright ſcarlet, and acquires ſomething ſo neceſſary to the exiſtence of life, that we can live ſcarcely a minute without this wonderful proceſs.

The analogy between the leaves of plants and the lungs or gills of animals ſeems to embrace ſo many circumſtances, that we can ſcarcely withhold our aſſent to their per⯑forming ſimilar offices.

1. The great ſurface of the leaves compared to that of the trunk and branches of trees is ſuch, that it would ſeem to be an organ well adapted for the purpoſe of expoſing the vegetable juices to the influence of the air; this however we ſhall ſee afterwards is pro⯑bably performed only by their upper ſurfaces, yet even in this caſe the ſurface of the leaves in general bear a greater proportion to the ſurface of the tree, than the lungs of animals to their external ſurfaces.

[102] 2. In the lungs of animal, the blood after having been expoſed to the air in the ex⯑tremities of pulmonary artery, is changed in colour from deep red to bright ſcarlet, and certainly in ſome of its eſſential properties; it is then collected by the pulmonary vein and returned to the heart. To ſhew a ſimilarity of circumſtance in the leaves of plants the following experiment was made, June 24, 1781: A ſtalk with leaves and ſeed-veſſels of large ſpurge (Euphorbia helioſcopia) had been ſeveral days placed in a decoction of madder (Rubia tinctorum) ſo that the lower part of the ſtem, and two of the undermoſt leaves were immerſed in it. After having waſhed the immerſed leaves in clear water, I could readily diſcern the colour of the madder paſſing along the middle rib of each leaf. This red artery was beautifully viſible both on the under and upper ſurface of the leaf; but on the upper ſide many red branches were ſeen going from it to the extremities of the leaf, which on the other ſide were not viſible except by looking through it againſt the light. On this under ſide a ſyſtem of branching veſſels carrying a pale milky fluid were ſeen coming from the extremities of the leaf, and covering the whole underſide of it, and joining into two large veins, one on each ſide of the red artery in the middle rib of the leaf, and along with it deſcending to the footſtalk or petiole. On ſlitting one of theſe leaves with ſciſſars, and having a common magnifying lens ready, the milky blood was ſeen oozing out of the returning veins on each ſide of the red artery in the middle rib, but none of the red fluid from the artery.

All theſe appearances were more eaſily ſeen in a leaf of Picris treated in the ſame manner; for in this milky plant the ſtems and middle rib of the leaves are ſometimes naturally coloured reddiſh, and hence the colour of the madder ſeemed to paſs further into the ramifications of their leaf-arteries, and was there beautifully viſible with the returning branches of milky veins on each ſide.

3. From theſe experiments the upper ſurface of the leaf appeared to be the immediate organ of reſpiration, becauſe the coloured fluid was carried to the extremities of the leaf by veſſels moſt conſpicuous on the upper ſurface, and there changed into a milky fluid, which is the blood of the plant, and then returned by concomitant veins on the under ſurface, which were ſeen to ooze when divided with ſciſſars, and which in Picris, parti⯑cularly render the under ſurface of the leaves greatly whiter than the upper one.

4. As the upper ſurface of leaves conſtitutes the organ of reſpiration, on which the ſap is expoſed in the terminations of arteries beneath a thin pellicle to the action of the atmoſphere, theſe ſurfaces in many plants ſtrongly repel moiſture, as cabbage-leaves, whence the particles of rain lying over their ſurfaces without touching them, as obſerved by Mr. Melville (Eſſays Literary and Philoſop. Edinburgh) have the appearance of glo⯑bules of quickſilver. And hence leaves laid with the upper ſurfaces on water, wither as ſoon as in the dry air, but continue green many days, if placed with the under ſurfaces on water, as appears in the experiments of Mons. Bonnet (Uſage des Fevilles.) Hence ſome aquatic plants, as the Water-lily (Nymphoea) have the lower ſides of their leaves floating on the water, while the upper ſurfaces remain dry in the air.

5. As thoſe inſects, which have many ſpiracula, or breathing apertures, as waſps and flies, are immediately ſuffocated by pouring oil upon them, I carefully covered with [103] oil the ſurfaces of ſeveral leaves of Phlomis, of Portugal Laurel, and Balſams, and though it would not regularly adhere, I found them all die in a day or two.

Of aquatic leaves, ſee Note on Trapa and on Fucus, in Vol. II. to which muſt be added that many leaves are furniſhed with muſcles about their footſtalks, to turn their upper ſurfaces to the air or light, as Mimoſa and Hedyſarum gyrans. From all theſe analogies I think there can be no doubt but that leaves of trees are their lungs, giving out a phlo⯑giſtic material to the atmoſphere, and abſorbing oxygene or vital air.

6. The great uſe of light to vegetation would appear from this theory to be by diſen⯑gaging vital air from the water which they perſpire, and thence to facilitate its union with their blood expoſed beneath the thin ſurface of their leaves; ſince when pure air is thus applied, it is probable, that it can be more readily abſorbed. Hence in the curious experiments of Dr. Prieſtley and Mr. Ingenhouze, ſome plants purified air leſs than others, that is, they perſpired leſs in the ſunſhine; and Mr. Scheele found that by putting peas into water, which about half-covered them, that they converted the vital air into fixed air, or carbonic acid gas, in the ſame manner as in animal reſpiration. See Note XXXIV.

7. The circulation in the lungs or leaves of plants is very ſimilar to that of fiſh. In fiſh the blood after having paſſed through their gills does not return to the heart as from the lungs of air-breathing animals, but the pulmonary vein taking the ſtructure of an artery after having received the blood from the gills, which there gains a more florrid colour, diſtributes it to the other parts of their bodies. The ſame ſtructure occurs in the livers of fiſh, whence we ſee in thoſe animals two circulations independent of the power of the heart, viz. that beginning at the termination of the veins of the gills, and branching through the muſcles; and that which paſſes through the liver; both which are carried on by the action of thoſe reſpective arteries and veins. Monro's Phyſiology of Fiſh, p. 19.

The courſe of the fluids in the roots, leaves, and buds of vegetables ſeems to be per⯑formed in a manner ſimilar to both theſe. Firſt the abſorbent veſſels of the roots and ſurfaces unite at the footſtalk of the leaf; and then, like the Vena Portarum, an artery commences without the intervention of a heart, and ſpreads the ſap in its numerous rami⯑fications on the upper ſurface of the leaf; here it changes its colour and properties, and becomes vegetable blood; and is again collected by a pulmonary vein on the under ſur⯑face of the leaf. This vein, like that which receives the blood from the gills of fiſh, aſſumes the office and name of an artery, and branching again diſperſes the blood up⯑ward to the bud from the footſtalk of the leaf, and downward to the roots; where it is all expended in the various ſecretions, the nouriſhment and growth of the plant, as faſt as it is prepared.

II. The organ of reſpiration already ſpoken of belongs particularly to the ſhoots or buds, but there is another pulmonary ſyſtem, perhaps totally independent of the green foliage, which belongs to the fructification only, I mean the corol or petals. In this there is an artery belonging to each petal, which conveys the vegetable blood to its ex⯑tremities, expoſing it to the light and air under a delicate membrane covering the internal ſurface of the petal, where it often changes its colour, as is beautifully ſeen in ſome party-coloured [104] poppies; though it is probable ſome of the irideſcent colours of flowers may be owing to the different degrees of tenuity of the exterior membrane of the leaf refracting the light like ſoap-bubbles, the vegetable blood is then returned by correſpondent vege⯑table veins, exactly as in the green foliage; for the purpoſes of the important ſecretions of honey, wax, the finer eſſential oil, and the prolific duſt of the anthers.

1. The vaſcular ſtructure of the corol as above deſcribed, and which is viſible to the naked eye, and its expoſing the vegetable juices to the air and light during the day, evinces that it is a pulmonary organ.

2. As the glands which produce the prolific duſt of the anthers, the honey, wax, and frequently ſome odoriferous eſſential oil, are generally attached to the corol, and always fall off and periſh with it, it is evident that the blood is elaborated or oxygenated in this pulmonary ſyſtem for the purpoſe of theſe important ſecretions.

3. Many flowers, as the Colchicum, and Hamamelis ariſe naked in autumn, no green leaves appearing till the enſuing ſpring; and many others put forth their flowers and complete their impregnation early in the ſpring before the green foliage appears, as Mezereon, cherries, pears, which ſhews that theſe corols are the lungs belonging to the fructification.

4. This organ does not ſeem to have been neceſſary for the defence of the ſtamens and piſtils, ſince the calyx of many flowers, as Tragopogon, performs this office; and in many flowers theſe petals themſelves are ſo tender as to require being ſhut up in the calyx during the night, for what other uſe then can ſuch an apparatus of veſſels be deſigned?

5. In the Helleborus-niger, Chriſtmas-roſe, after the ſeeds are grown to a certain ſize, the nectaries and ſtamens drop off, and the beautiful large white petals change their colour to a deep green, and gradually thus become as calyx incloſing and defending the ripening ſeeds, hence it would ſeem that the white veſſels of the corol ſerved the office of expoſing the blood to the action of the air, for the purpoſes of ſeparating or producing the honey, wax, and prolific duſt, and when theſe were no longer wanted, that theſe veſſels coaleſced like the placental veſſels of animals after their birth, and thus ceaſed to perform that office and loſt at the ſame time their white colour. Why ſhould they looſe their white colour, unleſs they at the ſame time loſt ſome other property beſides that of defending the ſeed-veſſel, which they ſtill continue to defend?

6. From theſe obſervations I am led to doubt whether green leaves be abſolutely neceſſary to the progreſs of the fruit-bud after the laſt year's leaves are fallen off. The green leaves ſerve as lungs to the ſhoots and foſter the new buds in their boſoms, whether theſe buds be leaf-buds or fruit-buds; but in the early ſpring the fruit-buds expand their corols, which are their lungs, and ſeem no longer to require green leaves; hence the vine bears fruit at one joint without leaves, and puts out a leaf-bud at another joint without fruit. And I ſuppoſe the green leaves which riſe out of the earth in the ſpring from the Colchicum are for the purpoſe of producing the new bulb, and its pla⯑centa, and not for the giving maturity to the ſeed. When currant or gooſberry trees loſe their leaves by the depredation of inſects the fruit continues to be formed, though leſs ſweet and leſs in ſize.

[105] 7. From theſe facts it appears that the flower-bud after the corol falls off, (which is its lungs,) and the ſtamens and nectary along with it, becomes ſimply an uterus for the purpoſe of ſupplying the growing embryon with nouriſhment, together with a ſyſtem of abſorbent veſſels which bring the juices of the earth to the footſtalk of the fruit, and which there changes into an artery for the purpoſe of diſtributing the ſap for the ſecretion of the ſaccharine or farinaceous or aceſcent materials for the uſe of the embryon. At the ſame time as all the veſſels of the different buds of trees inoſculate or communicate with each other, the fruit becomes ſweeter and larger when the green leaves continue on the tree, but the mature flowers themſelves, (the ſucceeding fruit not conſidered) perhaps ſuffer little injury from the green leaves being taken off, as ſome floriſts have obſerved.

8. That the veſſels of different vegetable buds inoſculate in various parts of their circulation is rendered probable by the increaſed growth of one bud, when others in its vicinity are cut away; as it thus ſeems to receive the nouriſhment which was before divided amongſt many.

NOTE XXXVIII.—VEGETABLE IMPREGNATION.

FROM the accurate experiments and obſervations of Spallanzani it appears that in the Spartium Junceum, ruſh-broom, the very minute ſeeds were diſcerned in the pod at leaſt twenty days before the flower is in full bloom, that is twenty days before fecunda⯑tion. At this time alſo the powder of the anthers was viſible, but glued faſt to their ſum⯑mits. The ſeeds however at this time, and for ten days after the bloſſom had fallen off, appeared to conſiſt of a gelatinous ſubſtance. On the eleventh day after the falling of the bloſſom the ſeeds became heart-ſhape, with the baſis attached by an appendage to the pod, and a white point at the apex; this white point was on preſſure found to be a cavity in⯑cluding a drop of liquor.

On the 25th day the cavity which at firſt appeared at the apex was much enlarged and ſtill full of liquor, it alſo contained a very ſmall ſemi-tranſparent body, of a yellowiſh colour, gelatinous, and fixed by its two oppoſite ends to the ſides of the cavity.

In a month the ſeed was much enlarged and its ſhape changed from a heart to a kidney, the little body contained in the cavity was increaſed in bulk and was leſs tranſparent, and gelatinous, but there yet appeared no organization.

[106] On the 40th day the cavity now grown larger was quite filled with the body, which was covered with a thin membrane; after this membrane was removed the body appeared of a bright green, and was eaſily divided by the point of a needle into two portions, which manifeſtly formed the two lobes, and within theſe attached to the lower part the exceedingly ſmall plantule was eaſily perceived.

The foregoing obſervations evince, 1. That the ſeeds exiſt in the ovarium many days before fecundation. 2. That they remain for ſome time ſolid, and then a cavity containing a liquid is formed in them. 3. That after fecundation a body begins to appear within the cavity fixed by two points to the ſides, which in proceſs of time proves to be two lobes containing a plantule. 4. That the ripe ſeed conſiſts of two lobes adhering to a plantule, and ſurrounded by a thin membrane which is itſelf covered with a huſk or cuticle. Spalanzani's Diſſertations, Vol. II. p. 253.

The analogy between ſeeds and eggs has long been obſerved, and is confirmed by the mode of their production. The egg is known to be formed within the hen long before its impregnation; C. F. Wolf aſſerts that the yolk or the egg is nouriſhed by the veſſels of the mother, and that it has from thoſe its arterial and venous branches, but that after impregnation theſe veſſels gradually become impervious and obliterated, and that new ones are produced from the fetus and diſperſed into the yolk. Haller's Phyſiolog. Tom. VIII. p. 94. The young ſeed after fecundation, I ſuppoſe, is nouriſhed in a ſimilar manner from the gelatinous liquor, which is previouſly depoſited for that purpoſe; the uterus of the plant producing or ſecreting it into a reſervoir or amnios in which the embryon is lodged, and that the young embryon is furniſhed with veſſels to abſorb a part of it, as in the very early embryon in the animal uterus.

The ſpawn of frogs and of fiſh is delivered from the female before its impregnation. M. Bonnet ſays that the male ſalamander darts his ſemen into the water, where it forms a little whitiſh cloud which is afterwards received by the ſwoln anus of the female, and ſhe is fecundated.—He adds that marine plants approach near to theſe ani⯑mals, as the male does not project a fine powder but a liquor which in like manner forms a little cloud in the water.—And further adds, who knows but the powder of the ſtamina of certain plants may not make ſome impreſſion on certain germs belonging to the ani⯑mal kingdom! Letter XLIII. to Spalanzani, Oevres Philoſ.

Spalanzani found that the ſeminal fluid of frogs and dogs even when diluted with much water retained its prolific quality. Whether this quality be ſimply a ſtimulus exciting the egg into animal action, which may be called a vivifying principle, or whether part of it be actually conjoined with the egg is not yet determined, though the latter ſeems more probable from the frequent reſemblance of the fetus to the male parent. A conjunction however of both the male and female influence ſeems neceſſary for the purpoſe of reproduction throughout all organized nature, as well in hermaphrodite inſects, microſcopic animals, and polypi, and exiſts as well in the formation of the buds of vegetables as in the production of their ſeeds, which is ingeniouſly conceived and explained by Linneus. After having compared the flower to the larva of a butterfly, [107] conſiſting of petals inſtead of wings, calyxes inſtead of wing-ſheaths, with the organs of reproduction, and having ſhewn the uſe of the farina in fecundating the egg or ſeed, he proceeds to explain the production of the bud. The calyx of a flower, he ſays, is an expanſion of the outer bark, the petals proceed from the inner bark or rind, the ſtamens from the alburnum or woody circle, and the ſtyle from the pith. In the pro⯑duction and impregnation of the ſeed a commixture of the ſecretions of the ſtamens and ſtyle are neceſſary; and for the production of a bud he thinks the medulla or pith burſts its integuments and mixes with the woody part or alburnum, and theſe forcing their paſſage through the rind and bark conſtitute the bud or viviparous progeny of the vegetable. Syſtem of Vegetables tranſlated from Linneus, p. 8.

It has been ſuppoſed that the embryon vegetable after fecundation, by its living activity or ſtimulus exerted on the veſſels of the parent plant, may produce the fruit or ſeed-lobes, as the animal fetus produces its placenta, and as vegetable buds may be ſuppoſed to produce their umbilical veſſels or roots down the bark of the tree. This in reſpect to the production of the fruit ſurrounding the ſeeds of trees has been aſſimilated to the gall-nuts on oak-leaves, and to the bedeguar on briars, but there is a powerful objection to this doctrine, viz. that the fruit of figs, all which are female in this country, grow nearly as large without fecundation, and therefore the embryon has in them no ſelf-living principle.

NOTE XXXIX.—VEGETABLE GLANDULATION.

THE glands of vegetables which ſeparate from their blood the mucilage, ſtarch, or ſugar for the placentation or ſupport of their ſeeds, bulbs, and buds; or thoſe which depoſit their bitter, acrid, or narcotic juices for their defence from depredations of infects or larger animals; or thoſe which ſecrete reſins or wax for their protection from moiſture or froſts, conſiſt of veſſels too fine for the injection or abſorption of coloured fluids, and have not therefore yet been exhibited to the inſpection even of our glaſſes, and can therefore only be known by their effects, but one of the moſt curious and im⯑portant of all vegetable ſecretions, that of honey, is apparent to our naked eyes, though before the diſcoveries of Linneus the nectary or honey-gland had not even acquired a name.

[108] The odoriferous eſſential oils of ſeveral flowers ſeem to have been deſigned for their defence againſt the depredations of inſects, while their beautiful colours were a neceſſary conſequence of the ſize of the particles of their blood, or of the tenuity of the exterior membrane of the petal. The uſe of the prolific duſt is now well aſcertained, the wax which covers the anthers prevents this duſt from receiving moiſture, which would make it burſt prematurely and thence prevent its application to the ſtigma, as ſometimes happens in moiſt years and is the cauſe of deficient fecundation both of our fields and orchards.

The univerſality of the production of honey in the vegetable world, and the very complicated apparatus which nature has conſtructed in many flowers, as well as the acrid or deleterious juices ſhe has furniſhed thoſe flowers with (as in the Aconite) to protect this honey from rain and from the depredations of inſects, ſeem to imply that this fluid is of very great importance in the vegetable economy; and alſo that it was neceſſary to expoſe it to the open air previous to its reabſorption into the vegetable veſſels.

In the animal ſyſtem the lachrymal gland ſeparates its fluid into the open air for the purpoſe of moiſtening the eye, of this fluid the part which does not exhale is abſorbed by the puncta lachrymalia and carried into the noſtrils; but as this is not a nutritive fluid the analogy goes no further than its ſecretion into the open air and its reabſorption into the ſyſtem; every other ſecreted fluid in the animal body is in part abſorbed again into the ſyſtem, even thoſe which are eſteemed excrementitious, as the urine and perſpirable matter, of which the latter is ſecreted, like the honey, into the external air. That the honey is a nutritious fluid, perhaps the moſt ſo of any vegetable production, appears from its great ſimilarity to ſugar, and from its affording ſuſtenance to ſuch numbers of inſects, which live upon it ſolely during ſummer, and lay it up for their winter proviſion. Theſe proofs of its nutritive nature evince the neceſſity of its re⯑abſorption into the vegetable ſyſtem for ſome uſeful purpoſe.

This purpoſe however has as yet eſcaped the reſearches of philoſophical botaniſts. M. Pontedera believes it deſigned to lubricate the vegetable uterus, and compares the horn-like nectaries of ſome flowers to the appendicle of the caecum inteſtinum of animals. Antholog. p. 49.) Others have ſuppoſed that the honey, when reabſorbed, might ſerve the purpoſe of the liquor amnii, or white of the egg, as a nutriment for the young embryon or fecundated ſeed in its early ſtate of exiſtence. But as the nectary is found equally general in male flowers as in female ones; and as the young embryon or ſeed grows before the petals and nectary are expanded, and after they fall off; and, thirdly, as the nectary ſo ſoon falls off after the fecundation of the piſtillum; theſe ſeem to be inſurmountable objections to both the above-mentioned opinions.

In this ſtate of uncertainty conjectures may be of uſe ſo far as they lead to further experiment and inveſtigation. In many tribes of inſects, as the ſilk-worm, and perhaps in all the moths and butterflies, the male and female parents die as ſoon as the eggs are [109] impregnated and excluded; the eggs remaining to be perfected and hatched at ſome future time. The ſame thing happens in regard to the male and female parts of flowers; the anthers and filaments, which conſtitute the male parts of the flower, and the ſtigma and ſtyle, which conſtitute the female part of the flower, fall off and die as ſoon as the ſeeds are impregnated, and along with theſe the petals and nectary. Now the moths and butterflies above-mentioned, as ſoon as they acquire the paſſion and the apparatus for the reproduction of their ſpecies, looſe the power of feeding upon leaves as they did before, and become nouriſhed by what?—by honey alone.

Hence we acquire a ſtrong analogy for the uſe of the nectary or ſecretion of honey in the vegetable economy, which is, that the male parts of flowers, and the female parts, as ſoon as they leave their fetus-ſtate, expanding their petals, (which conſtitute their lungs,) become ſenſible to the paſſion, and gain the apparatus for the reproduction of their ſpecies, and are fed and nouriſhed with honey like the inſects above deſcribed; and that hence the nectary begins its office of producing honey, and dies or ceaſes to produce honey at the ſame time with the birth and death of the ſtamens and the piſtils; which, whether exiſting in the ſame or in different flowers, are ſeparate and diſtinct animated beings.

Previous to this time the anthers with their filaments, and the ſtigmas with their ſtyles, are in their fetus-ſtate ſuſtained by their placental veſſels, like the unexpanded leaf-bud; with the ſeeds exiſting in the vegetable womb yet unimpregnated, and the duſt yet unripe in the cells of the anthers. After this period they expand their petals, which have been ſhewn above to conſtitute the lungs of the flower; the placental veſſels, which before nouriſhed the anthers and the ſtigmas, coaleſce or ceaſe to nouriſh them; and they now acquire blood more oxygenated by the air, obtain the paſſion and power of reproduction, are ſenſible to heat, and cold, and moiſture, and to mechanic ſtimulus, and become in reality inſects fed with honey, ſimilar in every reſpect except their being attached to the tree on which they were produced.

Some experiments I have made this ſummer by cutting out the nectaries of ſeveral flowers of the aconites before the petals were open, or had become much coloured, ſome of theſe flowers near the ſummit of the plants produced no ſeeds, others lower down produced ſeeds; but they were not ſufficiently guarded from the farina of the flowers in their vicinity; nor have I had opportunity to try if theſe ſeeds would vegetate.

I am acquainted with a philoſopher, who contemplating this ſubject thinks it not impoſſible, that the firſt inſects were the anthers or ſtigmas of flowers; which had-by ſome means looſed themſelves from their parent plant, like the male flowers of Valliſneria; and that many other inſects have gradually in long proceſs of time been formed from theſe; ſome acquiring wings, others fins, and others claws, from their ceaſeleſs efforts to procure their food, or to ſecure themſelves from injury. He contends, that none of theſe changes are more incomprehenſible than the tranſformation of tadpoles into frogs, and caterpillars into butterflies.

There are parts of animal bodies, which do not require oxygenated blood for the purpoſe of their ſecretions, as the liver; which for the production of bile takes its blood [110] from the meſenteric veins, after it muſt have loſt the whole or a great part of its oxyge⯑nation, which it had acquired in its paſſage through the lungs. In like manner the pericarpium, or womb of the flower, continues to ſecrete its proper juices for the preſent nouriſhment of the newly animated embryon-ſeed; and the ſaccharine, aceſcent, or ſtarchy matter of the fruit or ſeed-lobes for its future growth; in the ſame manner as theſe things went on before fecundation; that is, without any circulation of juices in the petals, or production of honey in the nectary; theſe having periſhed and fallen off with the male and female apparatus for impregnation.

It is probable that the depredations of inſects on this nutritious fluid muſt be injurious to the products of vegetation, and would be much more ſo, but that the plants have either acquired means to defend their honey in part, or have learned to make more than is abſolutely neceſſary for their own economy. In the ſame manner the honey-dew on trees is very injurions to them; in which diſeaſe the nutritive fluid, the vegetable-ſap-juice, ſeems to be exſuded by a retrograde motion of the cutaneous lymphatics, as in the ſweating ſickneſs of the laſt century. To prevent the depredation of inſects on honey a wealthy man in Italy is ſaid to have poiſoned his neighbour's bees perhaps by mixing arſnic with honey, againſt which there is a moſt flowery declamation in Quintilian. No. XIII. As the uſe of the wax is to preſerve the duſt of the anthers from moiſture, which would prematurely burſt them, the bees which collect this for the conſtruction of the combs or cells, muſt on this account alſo injure the vegetation of a country where they too much abound.

It is not eaſy to conjecture why it was neceſſary that this ſecretion of honey ſhould be expoſed to the open air in the nectary or honey-cup, for which purpoſe of great an apparatus for its defence from inſects and from ſhowers became neceſſary. This diffi⯑culty increaſes when we recollect that the ſugar in the joints of graſs, in the ſugar-cane, and in the roots of beets, and in ripe fruits is produced without the expoſure to the air. On ſuppoſition of its ſerving for nutriment to the anthers and ſtigmas it may thus acquire greater oxygenation for the purpoſe of producing greater powers of ſenſibility, ac⯑cording to a doctrine lately advanced by a French philoſopher, who has endeavoured to ſhew that the oxygene, or baſe of vital air, is the conſtituent principle of our power of ſenſibility.

From this proviſion of honey for the male and female parts of flowers, and from the proviſion of ſugar, ſtarch, oil, and mucilage, in the fruits, ſeed-cotyledons, roots, and buds of plants laid up for the nutriment of the expanding fetus, not only a very numerous claſs of inſects, but a great part of the larger animals procure their food; and thus enjoy life and pleaſure without producing pain to others, for theſe ſeeds or eggs with the nutriment laid up in them are not yet endued with ſenſitive life.

The ſecretions from various vegetable glands hardened in the air produce gums, reſins, and various kinds of ſaccharine, ſaponaceous, and wax-like ſubſtances, as the gum of cherry or plumb-trees, gum tragacanth from the aſtragalus tragacantha, camphor from the laurus camphora, elemi from amyris elemifera, aneme from hymenoea courbaril, turpentine from piſtacia terebinthus, balſam of Mecca from the buds of amyris opobal⯑ſamum, [111] branches of which are placed in the temples of the Eaſt on account of their fragrance, the wood is called xylobalſamum, and the fruit carpobalſamum; aloe from a plant of the ſame name; myrrh from a plant not yet deſcribed; the remarkably elaſtic reſin is brought into Europe principally in the form of flaſks, which look like black leather, and are wonderfully elaſtic, and not penetrable by water, rectified ether diſſolves it; its flexibility is encreaſed by warmth and deſtroyed by cold; the tree which yields this juice is the jatropha elaſtica, it grows in Guaiana and the neighbouring tracts of America; its juice is ſaid to reſemble wax in becoming ſoft by heat, but that it acquires no elaſticity till that property is communicated to it by a ſecret art, after which it is poured into moulds and well dried and can no longer be rendered fluid by heat. Mr. de la Borde phyſician at Cayenne has given this account. Manna is obtained at Naples from the fraxinus ornus, or manna-aſh, it partly iſſues ſpontaneouſly, which is preferred, and partly exſudes from wounds made purpoſely in the month of Auguſt, many other plants yield manna more ſparingly; ſugar is properly made from the ſaccharum officinale, or ſugar-cane, but is found in the roots of beet and many other plants; American wax is obtained from the myrica cerifera, candle-berry myrtle, the berries are boiled in water and a green wax ſeparates, with luke warm water the wax is yellow: the ſeed of croton ſebiferum are lodged in tallow; there are many other vegetable exſudations uſed in the various arts of dyeing, varniſhing, tanning, lacquering, and which ſupply the ſhop of the druggiſt with medicines and with poiſons.

There is another analogy, which would ſeem to aſſociate plants with animals, and which perhaps belongs to this Note on Glandulation, I mean the ſimilarity of their digeſtive powers. In the roots of growing vegetables, as in the proceſs of making malt, the farinaceous part of the ſeed is converted into ſugar by the vegetable power of digeſtion in the ſame manner as the farinaceous matter of ſeeds are converted into ſweet chyle by the animal digeſtion. The ſap-juice which riſes in the vernal months from the roots of trees through the alburnum or ſap-wood, owes its ſweetneſs I ſuppoſe to a ſimilar digeſtive power of the abſorbent ſyſtem of the young buds. This exiſts in many vegetables in great abundance as in vines, ſycamore, birch, and moſt abundantly in the palm-tree, (Iſert's Voyage to Guinea,) and ſeems to be a ſimilar fluid in all plants, as chyle is ſimilar in all animals.

Hence as the digeſted food of vegetables conſiſts principally of ſugar, and from that is produced again their mucilage, ſtarch, and oil, and ſince animals are ſuſtained by theſe vegetable productions, it would ſeem that the ſugar-making proceſs carried on in vegetable veſſels was the great ſource of life to all organized beings. And that if our improved chemiſtry ſhould ever diſcover the art of making ſugar from foſſile or aerial matter without the aſſiſtance of vegetation, food for animals would then become as plentiful as water, and mankind might live upon the earth as thick as blades of graſs, with no reſtraint to their numbers but the want of local room.

It would ſeem that roots fixed in the earth, and leaves innumerable waving in the air were neceſſary forthe decompoſition of water, and the converſion of it into ſaccharine [112] matter, which would have been not only cumberous but totally incompatible with the locomotion of animal bodies. For how could a man or quadruped have carried on his head or back a foreſt of leaves, or have had long branching lacteal or abſorbent veſſels terminating in the earth? Animals therefore ſubſiſt on vegetables; that is, they take the matter ſo far prepared, and have organs to prepare it further for the purpoſes of higher animation, and greater ſenſibility. In the ſame manner the apparatus of green leaves and long roots were found inconvenient for the more animated and ſenſitive parts of vegetable-flowers, I mean the anthers and ſtigmas, which are therefore ſeparate beings, endued with the paſſion and power of reproduction, with lungs of their own, and fed with honey, a food ready prepared by the long roots and green leaves of the plant, and pre⯑ſented to their abſorbent mouths.

From this outline a philoſopher may catch a glimpſe of the general economy of na⯑ture; and like the mariner caſt upon an unknown ſhore, who rejoiced when he ſaw the print of a human foot upon the ſand, he may cry out with rapture, "A GOD DWELLS HERE."

The following is an Addition to the Note on Coal, No. XXIII.

FROM this account of the production of coals from moraſſes it would appear, that coal-beds are not to be expected beneath maſſes of lime-ſtone. Nevertheleſs I have been lately informed by my friend Mr. Michell of Thornhill, who I hope will ſoon favour the public with his geological inveſtigations, that the beds of chalk are the uppermoſt of all the limeſtones; and that they reſt on the granulated limeſtone, called ketton-ſtone; which I ſuppoſe is ſimilar to that which covers the whole country from Leadenham to Sleaford, and from Sleaford to Lincoln; and that, thirdly, coal-delphs are frequently found beneath theſe two uppermoſt beds of limeſtone.

Now as the beds of chalk and of granulated limeſtone may have been formed by alluviation, on or beneath the ſhores of the ſea, or in vallies of the land; it would ſeem, that ſome coal countries, which in the great commotions of the earth had been ſunk beneath the water, were thus covered with alluvial limeſtone, as well as others with alluvial baſaltes, or common gravel-beds. Very extenſive plains which now conſiſt of alluvial materials, were in the early times covered with water; which has ſince diminiſhed, as the ſolid parts of the earth have increaſed. For the ſolid parts of the earth conſiſting chiefly of animal and vegetable recrements muſt have originally been formed or pro⯑duced from the water by animal and vegetable proceſſes; and as the ſolid parts of the earth may be ſuppoſed to be thrice as heavy as water, it follows that thrice the quantity of water muſt have vaniſhed compared with the quantity of earth thus produced. This may account for many immenſe beds of alluvial materials, as gravel, rounded ſand, granulated limeſtone, and chalk, covering ſuch extenſive plains as Lincoln-heath, having become dry without the ſuppoſition of their having been again elevated from the ocean. At the ſame time we acquire the knowledge of one of the uſes or final cauſes of the organized world, not indeed very flattering to our vanity, that it converts water into earth, forming iſlands and continents by its recrements or exuviae.