« PreviousContinue »
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 fubje&t; and leave it only proved, that was 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.]
Curious and important dif covery was announced to the world in the fixty-fifth vo Jume 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 strong objection. 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 circumitances 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 den◄ fity; and, in this proportion, they communicate more or lefs 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 fcald a perfon's hand, which can support the heat of air, of the fame tempe rature. And thus perhaps the wea ther, when hazy and loaded with vapour, feems to our feeling, hot ter than when pure and rare; al though 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 moift air. But no
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 rednefs 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 tranfmiffion 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 kin 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. 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, rife to the furface to refresh themselves 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 influence on the human body, the evaporation from the furface of their bodies, and the coldness 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 Ꮮ 4
.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 al ways 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, to 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 acceffion from without."
AN ESSAY on the ASCENT of VAPOUR, By ALEXANDER EASON, M. D.
[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 fupported by Lord Kaimes and Dr. Hamilton, 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 fteam 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,
ated with Field
and it would only us, what it could not retain
mofphere is electrified, but much ftronger in frofty, than in warm weather, and by no means lefs in
4. It is univerfally allowed, the night than in the day. it is likeds converting water into vapour, low places. From these facts we
contributes very much to- wife ftronger in elevated than in
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 state, 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 ftill 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 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.
"It likewife appears, that the 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 up. per 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 carried ftill higher, by the electric mat
"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 electrified. 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
ter in the upper regions. This, perhaps, is the caufe, why the air is fo clear and transparent in warm climates.
"By making fome obfervations on the falling of rain, we fhall have other proofs, that the electric matter is the great caufe by which vapour is fupported in the atmosphere. Here I muft obferve a fact, well known to all prefent, that bodies electrified, by the fame electric power (no matter whether pofitive or negative) repel each other; and, when electrified by the different powers, that is, the one plus and the other minus, attract each other: on coming into contact, an equilibrium is restored, and neither of them will fhew any figns of electricity.
"From this it follows: if two clouds are electrified by the fame power, they will repel each other, and the vapour be fufpended in both; but, when one is pofitive and the other negative, they will attract each other, and restore an equilibrium. The electric power, by which the vapour was fufpended, being now destroyed by the mutual action of the clouds on each other, the particles of water will have an opportunity of running together into each other, and, as they augment in fize, will gain a greater degree of gravity, defcending in fmall rain, or a heavy fhower, according to circumstances.
"A cloud, highly electrified, paffing over a high building or mountain, may be attracted by, and be deprived of its electricity, with out or with a violent explosion of thunder. If the cloud is electrified plus, the fire will defcend from the cloud to the mountain; but, if it be electrified minus, the fire will afçend from the mountain to the cloud. In both cafes, the effect is
the fame, and generally, heavy rain immediately, or foon after, follows: this is well known to the inhabitants of, and travellers among, mountains.
"From this we can eafily account, why thunder-fhowers are of ten partial, falling near, or among mountains, and the rain in fuch quantities, as to occafion rivers to be overflowed; whilft, at the diftance of a few miles, the ground continues parched up with drought, and the roads covered with duft.
"It often happens, that one clap of thunder is not fufficient to produce rain from a cloud, nor even a fecond: in fhort, the claps must be repeated, till an equilibrium is reftored, and then the rain muft, of confequence, fall. Sometimes we may have violent thunder and lightning without rain, and the black appearance of the heavens may be changed to a clear tranfparent iky, efpecially in warm weather. To account for this, it must be remembered, as I lately faid, that one or more claps of thunder are not always fufficient to produce rain from the clouds: fo, if an equilibrium be not restored, little or no rain will fall, and in a fhort time the electric matter, paffing from the earth to the clouds, or the fuperabundant quantity in the air, will electrify thofe black clouds, by which means the particles of vapour will be expanded, raised higher, and the air become clear. Clouds may be melted away, even when we are looking at them, by another caufe, that is, by the heat of the fun. We know, that tranfparent bodies are not heated by the fun, but opaque ones are ; the clouds being opaque bodies, are warmed by the rays of the fun thining on them, and any additional quantity of heat will rarify the vapour, and occafion its expanding in