This doctrine concerning the nature of fire is here ingenioufly applied to that curious fubject, the production of animal heat. i am inclined to think, fays the Author, that the fluid fire, as well as the fluid air, is attracted by plants in their growth, and becomes confolidated with the other materials of which they are formed, and makes a great part of their fubftance: that when they come to be digefted, and to fuffer in the veffels a kind of fermentation, part of the fire, as well as part of the air, recovers its fluid active state again, and diffuses itself in the body, digefting and feparating it: that the fire fo reproduced, by digeftion and feparation, continually leaving the body, its place is fupplied by freth quantities, arifing from the continual feparation that whatever quickens the motion of the fluids in an animal, quickens the feparation, and reproduces more of the fire; as exercife, &c.—Thus I imagine that animal heat arifes by or from a kind of fermentation in the juices of the body, in the fame manner as heat arifes in the liquors preparing for diftillation, wherein there is a feparation of the fpirituous from the watry and earthy parts.And it is remarkable that the liquor in a diftiller's vat, when in its highest and best state of fermentation, as I have been informed, has the fame degree of heat with the human body; that is, about 94 or 96.' Thus, as by a conftant fupply of fuel in a chimney, you keep a warm room, fo, by a conftant fupply of food in the ftomach, you keep a warm body.'

In the preceding quotation the Author very properly, in our opinion, qualifies that procefs, by which he fuppofes animal heat to be produced, by terming it only a kind of fermentation; although, by his allufion to the equal degree of heat produced in that carried on in a diftiller's vat, he may be thought to fuppose the two proceffes to be of a fimilar kind. Various have been the hypothefes that have been formed on this fubject, which neverthelefs ftill remains involved in confiderable obfcurity; as, of the many known mechanical or chemical procelles which have been affigned as the probable caufes productive of animal heat, fome cannot poffibly be carried on in the animal body, nor others, with fuch activity, as to generate a heat equal to that of a living animal. Paffing over the calidum innatum of the great father of phyfic with a refpectful and reverential filence, we fhall obferve that the modern theories which fuppofe animal heat to proceed from the mechanical attrition of the particles of the circulating fluids amongft each other, or against the fides of the veffels, although adopted by fome of the greatest phyfioloifts of this age, are evidently founded on a few deceitful experiments, in which the effects of chemical action have been attributed to mechanical attrition; as in the inftance of cream becoming hot in the act of churning, &c. The most plaufible

plaufible theory yet offered, is that which fuppofes it to be the product of fermentation or putrefaction. But we might ask those who maintain this opinion, what fermentatory or putrefactive process (if we affix the precife ideas to those terms which ufually are, and ought to be, annexed to them) can poffibly be carried on in the bodies of men or animals, however defended from the cold by natural or artificial cloathings of furs and other non-conducting materials, in fuch a climate as that of Siberia, for instance; where the thermometer (Farenheit's) frequently standing at 90 or 100 degrees below o, a heat is nevertheless generated, in the body of a man using a proper degree of exercise, equal at least to 90 degrees above that mark; and even the blood of whales and porpuffes fwimming in the lefs cold, indeed, but denfer and more perfectly conducting element of water, is known to be poffeffed of a degree of heat fenfibly fuperior even to that of a healthy man. We believe that there is not any fubftance, however prone to fermentation or putrefaction, which is known either to ferment or putrefy, and generate heat under fuch circumftances.But to view this matter in another light:

Here is a fermentatory or putrefactive process supposed to be carried on in the body, which generates greater quantities of heat, in proportion to the increafing degrees of cold in the medium in which the process is carried on, through a very confiderable extent of the fcale downwards. The human body, whether placed under the polar circle or under the line, poffeffes a degree of heat fomewhere between 90 and 100 degrees of Fahrenheit's thermometer. Let us fuppofe it, at a medium, to be 95°. When the thermometer ftands at temperate, or at 50°, a heat is generated within the body equal to 45°, which we may confider as added to the 50 which the body would poffefs, in the ftate of death. When water begins to freeze, and the thermometer ftands at 32°, the body, under proper exercise, continues as warm as before, and confequently produces no less than 63 additional degrees of heat. Expofed to the intense cold indicated when the thermometer ftands at o, it generates 32 degrees more, or 95 degrees in the whole; and in the enormous cold of 90° below o, the body of a living man produces 185 degrees of heat above that of a dead man expofed in the fame fituation: But is there any fermentative or putrefactive procefs, or any putrid ferment whatever yet known which operates in this manner? We do not mean to deny however, that, in the coldeft as well as in the warmeft climates, a fermentatory process is carried on in the ftomach and firft paffages of animals; that their juices manifeft a putrefcent difpofition; and that these inteftine motions are attended with heat: but they are so far from being capable of producing the heat which

the

the body enjoys in the coldest climates, that they appear, on the contrary, to be indebted to that very heat for their existence.

These observations, however, do not affect the Author's general theory concerning the nature of fire, which is perfectly reconcileable to any of the abovementioned fyftems. Till more light can be obtained on this fubject, it is fufficient only to fay that there exifts in the bodies of men and animals, during life, a certain calorific process, fui generis, [we leave to future inquirers to afcertain its nature, and to give it a more specific title] connected, in fome manner or another, with the organization, motion, or chemical properties of their constituent parts; by means of which the particles of fixed fire, refiding in their food and in their fubftance, are fucceffively extricated and rendered fluid, and are thereby brought into a condition of imparting warmth to the whole mafs; and that this process differs from the two laft-mentioned, in this effential particular, that it is brifkly carried on in the bodies of living animals, under degrees of cold much fuperior to thofe in which all fermentation and putrefaction of inanimate matters cease.

Towards the end of the letter the ingenious Author touches on another curious queftion relative to fire, and inquires whence the fudden and extraordinary degree of cold, perceptible on mixing certain chemical liquors, or, particularly, on the mixture of common falt and fnow, arifes. Though his manner of confidering various fubftances, as poffeffing different conducting powers with regard to fire, places this matter in a new point of view, and throws additional light on the nature of this element, and on the relations of different bodies towards it, it does not, however, appear to us applicable to the elucidation of the prefent fubject, in which he employs it. The Doctor proceeds analytically through the phenomena of this laft-mentioned experiment, and argues thus:

If the quantity of fire, fays he, before contained or diffused in the fnow and falt, was expelled in the uniting of the two matters, it must be driven away, either thro' the air, or the veffel containing them. If it is driven off through the air, it must warm the air; and a thermometer held over the mixture, without touching it, would difcover the heat, by the rifing of the mercury, as it muft and always does in warm air..

This, indeed, I have not tried; but I fhould guess it would rather be driven off through the veffel, especially if the

We fhall ftop to take notice that M Geoffroy long ago obferved that, in the frigorific mixture of oil of vitriol with fal ammoniac, the copious fumes, which arife during the effervefcence, fenfibly raise the thermometer fufpended above the mixture. We shall not enquire whether, in this inftance, the effect is produced by the particles of fire flying off from the mixture, or arifes from other caufes which

7

veffel be metal, as being a better conductor than air; and so one fhould find the bafon warmer after fuch mixture. But, on the contrary, the veffel grows cold, and even water in which the veffel is fometimes placed for the experiment, freezes into hard: ice on the balon. Now I know not how to account for this, otherwife than by fuppofing that the compofition is a betterconductor of fire than the ingredients feparately,—and has a ftronger power of attracting fire, and does accordingly attractit fuddenly from the fingers, or a thermometer put into it, from the bafon that contains it, and from the water in contact with the outfide of the bafon; fo that the fingers have the fenfation of extreme cold, by being deprived of much of their natural fire; the thermometer finks, by having part of its fire drawn out of the mercury; the bafon grows colder to the touch, as by having its fire drawn into the mixture, it is become more capable of drawing and receiving it from the hand; and through the bafon, the water lofes its fire that kept it fluid, fo it becomes icc.-One would expect that from all this attracted acquifition of fire to the compofition, it fhould become warmer; and, in fact, the fnow and falt diffolve at the fame time into water, without freezing.'

This manner of accounting for the phenomena in this experiment violently thakes, by implication at leaft, the credit of the thermometer; for if that inftrument, in this inftance, defcends on being brought into contact with bodies not colder than itself, we may be deceived by trusting to its report in other inftances. As we are loth, however, to entertain fufpicions of the veracity of this ufeful inftrument, we fhall endeavour to re-establish the credit of its teftimony: and the ingenious Author, we are confident, will not be forry if we fucceed, though at the expence of his folution of the phenomenon; especially as we may collect from the whole tenor of his writings, that his philofophical opinions fit loofe, and with an eafy negligence, upon him, and as he has always been ready to exchange them for others which carry a greater appearance of verifimilitude.

That different bodies are more or less perfect conductors of fire than others, is rendered evident from the Author's observations, which may be confirmed by the perfonal experience of every man who wears a coat, though unconscious that he owes the prefervation of his heat to its non-conducting quality but the fuperior conducting power of one body to that of another confifts not, we apprehend, in its power of robbing a third body, a thermometer for inftance, of more of its fire, than an imper

which might be mentioned; as in the generality of frigorific mixtures, where much greater degrees of cold are produced, no such apIearance has been obferved.

fect

fect conductor of the fame temperature would; but in its depriving it of an equal portion of its fire in a fhorter time; as may be evinced from fome common experiments. Mercury, for inftance, is a better conductor than air. Let us suppose a bafon, containing the firft-mentioned fluid, to have remained a fufficient time in a room where the temperature of the air is 40 degrees. If a thermometer ftanding at 60° be brought into the room, and be immerfed in the mercury, this more perfect conductor of fire will cause it to descend to 40°, and no lower; but the thermometer will lofe thefe 20 degrees of heat in a very fbort time. Had the fame thermometer been fufpended in the air of the fame room, it would, by its communication with this lefs perfect conductor, have loft juft as many degrees of heat, though in a longer time. If we had fuppofed the temperature of this room to have been 60°, the thermometer immerfed in the mercury, notwithstanding the fuperior conducting power of that fubftance, would not have defcended at all. A better conductor therefore will not caufe the thermometer to fink, unless fuch conductor be colder than the thermometer. That inftrument therefore defcends in the mixture of fnow and falt, not because the compofition is a better conductor than either of the two fubftances fingly, but because it is colder than either of them t. The mixture undoubtedly, as the Author obferves, attracts fire from the bafon, and from the finger and thermometer immerfed in it; but, previously to its attracting their fire, it has, in fome manner or another, apparently lost its own, and, like other cold bodies, draws fire from the warmer bodies furrounding and in contact with it. We may add, that the melting of the fnow and falt into water without freezing, is not only an equivocal fign of the prefence of heat acquired during this procefs, but that this very diffolution, in the generality of frigorific mixtures, appears to be antecedent, and indifpenfably neceffary, to the production of the cold.-Let us next inquire whence the cold proceeds, and what becomes of the fire which difappears in this process.

We are diffident of fucceeding in our fearch after this fugitive element, which feems to have eluded the penetration of

+ It would carry us too far to apply the preceding reafoning particularly to the human body, or to other bodies poffeffed, like it, of a power of generating heat within themfelve, with regard to which this doctrine requires a particular modification. Such bodies will indeed lofe more beat from the contact of a cold perfect conductor, than from that of an imperfect one of the fame temperature; though a thermometer or other body, poffeffed only of a determinate, unrenewable portion of fire, can only lofe its excefs above the quantity contained in the cold body, whether the latter be a perfect conductor

or not.