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in the center of which I made a fmall perforation. As the light of the burning body efcaped through this perforation, I examined it with a prifin, and obferved the following appearances. When the fpirits of wine were fet on fire, all the rays appeared in the perforation; but the violet, the blue, and the green, in the greatest abundance. When the combustion of the fpirits was checked, by throwing fome fal ammoniac into the mixture, the red rays difappeared; but when, by the long continuance of the flame, the fal ammoniac was rendered fo hot as to increafe rather than diminish the combuftion, the red rays again appeared at the perforation. If the fcreen was ma naged fo that the different parts of the flame might be examined feparately, I always obferved that the colours varied according to the degree of heat. At the bafe of the flame, or where the heat was least, the indigo, the violet, and a very fmall tinge of the blue and green appeared. As I approached the vertex of the flame, the rays which efcaped became more and more numerous till I reached the top, when all the rays appeared in the prifm. It should be attended to, that when the red rays first made their appearance, their quantity was fmall, and gradually increafed as the eye in its examination approached that part where the heat was greateft. Mr. Melvill, when he made fome of the preceding experiments, obferved that the yellow rays frequently efcaped in the greatest abundance; but this fingularity proceeded from fome circumstances which efcaped his attention. In confequence of mixing acids or falts with the burning fpirits, a very denfe fume of unignited particles arifes, and before the rays of

the burning body arrive at the perforation where the prifm catches them, they muft pafs through a medium which will abforb a great part of the indigo and the violet. On the other hand, owing to the imperfection of the decompofition, very few of the red rays are feparated from their bafis, and confequently the yellow and the orange rays are thofe alone which pass through the unburnt smoke of the flame.

"I would now proceed with obferving, that, befides the increase or decrease of heat, there are other modes of retarding or accelerating the combuftion of bodies, by which alfo may be examined fome of the preceding illustrations.

"1. A candle burns most rapidly and brilliantly in dephlogiiticated air.

66 2. The blue colour of a fulphureous flame in pure air is changed into a dazzling white.

". The flame of inflammable air, when mixed with nitrous air, is green. It is white strongly tinged with the indigo and violet when mixed with common air; but when mixed with dephlogisticated air, or furrounded by it, the brilliancy of its flame is most fingularly beautiful,

"If the preceding facts prove that light, as an heterogeneous body, is gradually decompofed during combuftion; if they prove likewife, that the indigo rays efcape with the leaft heat, and the red with the greatest; I think we may rationally account for feveral fingulari ties in the colours of different flames. If a piece of paper, impregnated with a folution of copper in the nitrous acid, be fet on fire, the bottom and fides of the flame are always tinged with green. Now this flame is evidently in that weak

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ference of the weight of the air over the fcale could ever amount to the 32d of a grain. I have, how. ever, contrived an apparatus which is executing, in which this caufe of

fallacy will be totally removed. I fhall, therefore, reit at prefent the ftate of this part of the fubject; and leave it only proved, that wa ter gains weight on being frozen."

SOME REMARKS on the OPINION that the ANIMAL BODY poffeffes the POWER of generating COLD. BY GEORGE BELL, M. D.

[From the MEMOIRS of the Literary and Philofophical Society at Manchester.]

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Curious and important dif covery was announced to the world in the fixty-fifth volume of the Philofophical Tranf actions. We are there informed, that Dr. Fordyce and other gentlemen, feveral different times, went into a room, the air of which was heated to a degree far above that of the human blood; and though they remained there, fometimes for the fpace of half an hour, yet the heat of their bodies was not increased by more than 3 or 4 degrees. From hence they concluded, that the living body poffeffes a peculiar power of generating cold by fome occult operation. The experiments feem to have been made with fufficient accuracy; but the conclufion drawn from them is liable to ftrong objec tion. For, in forming it, feveral circumstances have been overlook ed, which, in my opinion, afford an eafy explanation of all the phenomena, on principles already known, without referring them to a new law of the animal body, which probably does not exift. Thefe circumftances I fhall endeavour to point out.

I. The first eaufe which pree vented their bodies from receiving a greater increase of heat was, The

rarefaction of the air with which they were furrounded.

"The quantity of heat which different fubftauces contain, is, in general, in proportion to their denfity; and, in this proportion, they communicate more or less of it to others. A cubical foot of water contains a much greater quantity of heat, than a cubical foot of air, of the fame temperature: and, if a third fubftance be added, its tempe rature will be confiderably changed by the hot water, while by the hot air it will hardly be changed in any perceptible degree. Many facts may be adduced, which ferve to il luftrate, and, at the fame time, are explained by this caufe. Thus, the fteam of boiling water will feald a perfon's hand, which can support the heat of air, of the fame temperature. And thus perhaps the wea ther, when hazy and loaded with` vapour, feems to our feeling, hotter than when pure and rare; although by the thermometer it is found to be equally warm in both inftances,

"This also was the true reason, why, in making thofe experiments, Dr. Fordyce always found that he could bear a greater degree of heat in dry, than in moist air. But no

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thing fhews more clearly the flownefs with which heat is imparted to a denfer fubftance, from one that is highly rarefied, than a circumstance mentioned in the paper in question: "that even the fmall quantity of mercury, contained in a thermometer which the gentlemen carried with them into the room, did not arrive at the degree to which the air was heated, during the whole time they remained there."

"II. Another caufe which, in the given fituation, would diminifh the effect of the heated air, is, The evaporation made from the furface of the body.

"That evaporation produces a confiderable abforption of heat, is well known and, in making the experiments, there is reafon to be lieve, that it took place in a confiderable degree. Dr. Fordyce, anxious perhaps to establish his general law, feems unwilling to allow its influence. But when it is confidered, that by the operation of the heat, the force of the circulation was increafed, the pores of the skin relaxed, and the preffure of the internal air diminished; when we are told, that a turgefcence of the veins, and an univerfal redness of the furface of the body, took place; we are compelled to refufe credit to the affertion, even of Dr. Fordyce, that there was no evaporation. The evaporation must have been great, and would diminish the effect of the external heat by furrounding the furface with a cool atmosphere, from its temperature fit for the abforption of heat, and from its rarity, unfit for the ready transmission of it into the body.

"III. But another very power ful caufe of the body's having preferved its temperature in the given fituation, remains to be noticed; which is, The fucceffive afflux of blood

to the furface, of a temperature inferior to that of the furrounding air. By this means the fmall quantity of heat which penetrated the ikin would be immediately carried off, and transferred throughout the body: and it would have required the fpace of many hours, before the whole mafs could have received any confiderable increase of heat.

"It has been adduced, in proof of the existence of the power of the living body to generate cold, that frogs, lizards, and other animals of the fame fort, poffefs it; for if touched, they feel cold. This proves only, that their heat is lefs than that of the hand, with which they are felt; and perhaps lefs than that of the air, when the trial is made.

"But it is extremely probable, that no animal whatever can live in health, for any confiderable time, in an atmosphere of a temperature fuperior in heat to that of its own blood. Thus we find, that the animals in queftion hide themselves in the day-time among thick grafs, where there is a great evaporation; and in places into which the rays of the fun cannot penetrate. Worms, in hot weather, during the day, lie deep in the ground; but in the night-time, when it is cool, rise to the furface to refresh themfelves in the dew. When frogs, worms, and fuch other animals, are expofed to air warmer than their blood, its influence is counteracted by the fame caufes which counteract its influ ence on the human body, the evaporation from the furface of their bodies, and the coldnefs of their blood. Such accidental expofure happens more frequently to them, than to the human fpecies; and, from the inferiority of their fize, they would be fooner heated through, and lefs able to refift the

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.noxious effects of the hot air, were not their power of refifting it made up in another refpect. In fuch fituations, the evaporation from the furface of their bodies is greater; for the fkin is more lax, and is alway's covered with moisture. It is, perhaps, for this purpose alfo, that it is rough and uneven; which, by extending the furface, caufes a greater evaporation.

Thefe may be faid to be the means through which the human body is preferved, in nearly the fame temperature, when it happens to be placed, for a time, in an atmofphere of a fuperior degree of heat. They feem to me fo adequate to this effect, that I would even venture to impute the increafe of the temperature of the body, from

96 to 100 degrees, which happened in the experiments, rather to the acceleration of the blood, than to the influx of heat from the external air. While the caufe of animal heat remains unknown, it would be prefumption to affert, that these are the only means by which the body is enabled to relift the effects of external heat. There may be others; and it is not unreasonable to fuppofe, that as external cold, perhaps by its tonic influence, increases the power of the body to generate heat, fo external heat may diminish that power, and thus leffen the quantity of heat generated within, while the evaporation, produced by the fame caufe, guards it againit receiving any acceflion from without."

AN ESSAY on the ASCENT of VAPOUR. By ALEXANDER EASON, M. D.

"THE

[ From the fame Publication. ]

HERE are few phenomena in nature, which have puzzled philofophers more, than the afcent of vapour: and the different theories laid down by doctors Halley and Defaguliers, have been rejected, while another, not lefs liable to objections, has been almost univerfally received.

This theory, which I fhall prefently mention, was at first invented by a French gentleman, Monfieur le Roi, and afterwards revived by Lord Kaimes, and doctor Hugh Hamilton. It is this that the air diffolves water, as water does faline fubftances: the folution being perfect, the air will become transparent.

Objections. 1. Were this theory true, evaporation could not be

performed without air; but Mr. Watt, contrary to the theory fup. ported by Lord Kaimes and Dr. Has milton, has proved, that when was ter in vacuo was boiled with a degree of heat very little greater than that of the human body, the steam came over, and was condenfed in the refrigeratory. But he relates, that the evaporation was not quicker than in the open air.

"2. Were the doctrine of folution true, the air would be heavier, the more water it contained; and, as clouds contain a great portion of water, they ought to float on the furface of the earth, and not in the higher regions, as we daily obferve,

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rated with water; and it would only yield to us, what it could not retain in folution.

4. It is univerfally allowed, that heat contributes very much towards converting water into vapour, which is again condenfed by cold. In what manner will the doctrine of folution account for the fpontaneous evaporation of water, and its being fufpended in air, in the coldest weather, even when the thermometer is below the freezing point? Though I cannot allow of fuch a folution as above mentioned, I can, however, readily admit of a strong attraction betwixt air and water: for no air is found without water, and no water without air.

mofphere is electrified, but much ftronger in frofty, than in warm weather, and by no means less in the night than in the day it is likewife ftronger in elevated than in low places. From thefe facts we may be enabled to account, why evaporation is carried on during very cold weather. All the heat contained in water, above what is fufficient to keep it in a fluid ftate, will convert it into vapour; which, in a north or north-east wind, when the electric matter greatly abounds, will be carried off with much rapi dity; and, by the power of electri city, will be rendered still lighter, the higher it afcends; each particle repelling each other, and prevent ing the cold from condenfing the vapour, in its afcent through the cold regions of the atmosphere. The higher it rifes, the more space there is for expanfion; and the more it is expanded, the clearer will the atmosphere appear, and, probably, the higher the mercury will rife in the barometer.

"Water, which is eight hundred times heavier than air, by a very fmall degree of heat may be converted into vapour, which vapour is one thoufand eight hundred times lighter than air, according to Mr. Watt. It confequently follows, that vapour will rife up in the atmofphere, to the height of its own fpecific gravity; but, long before" It likewife appears, that the it could reach to fo high a region, it would be condenfed by cold, and return to the earth in rain, were it not for the latent heat it contains, and the electric matter in the air.

electric matter is more fenfible near the furface of the earth, in cold northern countries, than in warm fouthern places. M. Volta, with a very fimple apparatus, on the upper gallery of St. Paul's, produced an electric fpark, which, he told me, in Italy, could not be done, but on a very high mountain, or in a fituation greatly elevated. This feems a wife provifion in nature, that the electric matter fhould ap pear near the furface of the earth in cold climates, to raise up and fufpend the vapour in the air, which otherways would be condensed by the cold; whereas, in warm countries, the heat of the earth will be fufficient to raise vapours to a great height, which are afterwards car

"Whatever I mention concerning electricity is from facts, and not from any theory written about it, which is above my comprehenfion. But as the terms now in ufe, viz. pofitive and negative, or plus and minus, are generally beft underftood, I fhall exprefs myfelf by them. The able Nollet has proved, that water electrified, will evaporate fafter, than water which is not elec. trified. Does is not follow, that the more electric matter is in the air, the quicker the evaporation of water will be? And Mr. Cavallo has proved, that at all times the at-ried ftill higher, by the electric mat

ter

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