HEAT

HEAT, in physiology, is used in a double sense, either as that peculiar sensation which is felt on the approach of burning bodies; or the cause of that sensation, in which latter sense it is synonymous with FIRE.

The disputes which were formerly carried on with regard to the nature of heat, namely, whether we were to account it merely an effect of motion in the parts of terrestrial bodies, or of a fluid per se, seem now to be generally determined in favour of the latter opinion. The electric fluid and elementary fire are commonly thought to be the same, for which opinion some proofs are adduced under the article ELECTRICITY. The

discovery made by Dr Black that heat is capable of remaining in many bodies in a latent state, also confirms this opinion considerably*. The consequence* See Evidences of this discovery, however, undoubtedly is, That what we call heat or fire, depends not on the presence or absence of that fluid distinguished by the name of elementary fire, but on its action; just as the phenomena of electricity depend not on the presence or absence of the electric fluid, which is equally present at all times and in all places, but on its action.—Another inference, easily deducible from this principle is, That as the electric fluid is by its different manner of action capable of producing two different and seemingly opposite effects, distinguished by the names of positive and negative electricity; so doth the same fluid, acting as elementary fire, produce two different and seemingly opposite effects, called heat and cold.—This assertion will.

will not be thought paradoxical, when we consider that ocular demonstration may be given us, that a substance (namely, vapour), exceedingly cold to the touch, shall yet contain as much heat, as would be sufficient to heat the water of which it is composed, red hot, did the nature of water permit it to endure an heat of this kind.

The philosophers of this country who have most recently treated the subject of heat scientifically are, Dr Martin of St Andrews, Dr Black of Edinburgh, and Dr Irvin of Glasgow. Dr Martin, in an essay on the various degrees of heat in bodies, endeavours to shew, that what we call heat, depends not entirely upon the quantity of elementary fire which is poured upon the body ab extra; but upon certain circumstances arising from the constitution of the body itself, and its situation with regard to others. "We all find (says he) our great heats to be in those places that lie low, and have a great height of atmosphere above them, and surrounded by eminences and rising grounds. If you ascend on high to the tops of very elevated mountains, you are chilled with cold, and you find everlasting snows, that after thousands of summers have scarce ever been thawed, though every day exposed to the direct rays of the sun, which in some places are darted perpendicularly upon them. So necessary, it seems, is a long and direct passage through all or the greatest part of the depth of our atmosphere, or the assistance of its pressure, or the reflections of rays from the earth's own surface, to invigorate those rays, and to give them strength for warming terrestrial bodies. To which too the particular sulphureous nature of the low parts of the atmosphere may not a little contribute.

"But what if the real solar heat, both in itself, and what it can communicate to us and other planetary bodies (while it is not concentrated by burning-glasses, or strengthened by other assistances), be vastly less than is commonly reckoned? All the natural heat we meet with here on the earth we are ready to ascribe to the heat of the sun, which perhaps has but a small share in it, overlooking a source of heat, which, though often spoke of by the theorists of the earth, is seldom considered in that advantageous light I would choose to take it. Every body has felt or heard, that the temperature of the air in mines or other places deep under ground is warm, or at least very tolerable; and we know from the nicest observations, that in the cave of the Observatory at Paris, only about 90 feet under ground, the heat keeps the thermometer at 53°; and that without any assistance from the sun, it being never sensibly increased by the most scorching seasons beyond its heat in the most severe winters that have been felt there.—And the same constant and unalterable degree of heat was observed by Mr Boyle, in a cave cut deep into the earth. And great and even troublesome heats are said to be observed at greater depths, and increasing in proportion to these depths; though I could wish that these heats had been more regularly measured and ascertained than what I find they have yet been. So that it would seem the body of the earth has a very great proper internal heat independent on the sun, and very much beyond what he without the intervention of our atmosphere could communicate to it; so great as, within 90 feet of its

surface, to raise the thermometer 93 divisions above Fahrenheit's cold mixture (of snow and spirit of nitre), or 453 divisions above what Mr Amontons reckoned the lowest degree of heat*. This heat of the earth at its surface is something less; and beyond that, its force decreases indeed very fast, so as to leave the air at a small height above it a good deal colder; and which we find on the very high hills to be excessively cold, and not to be much warmed by the additional heat of the sun's direct rays, if they be but little altered by the earth's surface and atmosphere.—So then the sun, though it be not the sole or chief fountain, is as it were the great regulator of motion, heat, and life, to the inhabitants of this system."

This opinion hath also been adopted by others, but will probably never come into general credit. We certainly perceive the sun to be the force and fountain of heat to the surface of the earth at least, as much as we perceive a common fire to be the source of heat to a person who stands before it. Nor is the sun's heat increased or diminished, except by those very circumstances which increase or diminish the heat of a common fire. Though a person should stand directly before the fire, yet if a strong blast of air rushes into the room at the same time, he will not find himself warm; but it would be a very erroneous conclusion to infer from thence, that the heat of the fire was less than what people commonly supposed. If he excludes the blast of air by shutting the room-door, he will find the heat greatly increased, though he comes no nearer the fire than before; and if he causes any substance capable of reflecting the light strongly to be placed at a small distance behind him, such as a large piece of tinned iron, he will then very probably feel the heat intolerable in that very place where it was disagreeably cold before. Now the circumstances which increase or diminish the heat of the sun are quite similar to those above-mentioned. On the top of an high mountain the air has free access on all sides, except the small point of earth where the person stands; the rest of the earth also is at too great a distance to have any effect in diminishing the cold by its reflective power; and as in these high regions there are commonly violent winds, the cold is thereby greatly increased. But when we descend into the plain, the air has much less access. The immense body of the earth effectually shelters us from the air on one side, at the same time that by its reflection it acts like the piece of metal abovementioned; while its inequalities in a great measure shelter us from the winds all around; and hence the heat at the foot of a mountain may be intolerable, while the cold is equally so at the top. It will now follow, that heat is not properly speaking the mere presence of the rays of the sun, but their action after a certain manner, without interruption from any other substance; and that cold is not any privation or absence of these rays, but is occasioned by whatever prevents them from acting in the manner abovementioned; and if there is any substance in nature which constantly tends to prevent that action, such a substance is cold in the abstract. A substance of this kind is our atmosphere, or some principle in it*; tho' very possibly that principle, as already hinted, may be no other than elementary fire itself acting in a different manner.

With regard to the opinions of Dr Black and Ir-
vine

vin upon this subject, it is impossible to give such a full account as could be wished; because the gentlemen have not thought proper to publish their sentiments to the world. From what we have been able to collect, however, and which is authenticated by Dr Black himself, his general opinion is, that heat is a substance per se: that when this substance is present in any terrestrial body, in a certain degree, that body is sensibly hot; when a lesser quantity of heat is present, the substance is cold; but there is no such thing as a positive cold; and all those degrees of cold known to us, even the most violent, are only smaller degrees of heat; and of this heat the sun is to us the only source and fountain. For his opinions concerning sensible and latent heat, see the articles CONGELATION, EVAPORATION, and FLUIDITY.

Dr Irvin hath attempted to give a solution of the phenomena of latent heat; and though he hath not yet published this explanation himself, an account of it hath appeared in an inaugural dissertation De igne, published at Edinburgh, by Dr Cleghorn, in 1779. The substance of what this gentleman hath delivered, is as follows:

"Heat is occasioned by a certain fluid, and not by motion alone, as some eminent writers have imagined: because, 1. Those who have adopted the hypothesis of motion, could never even prove the existence of that motion for which they contended; and though it should be granted, the phenomena could not from it be explained. 2. If heat depended on motion, it would instantaneously pass through an elastic body; but we see that heat passes through bodies slowly like a fluid. 3. If heat depended on vibration, it ought to be communicated from a given vibration in proportion to the quantity of matter, which is found not to hold true in fact. On the other hand, there are numberless arguments in favour of the opinion that heat proceeds from elementary fire. 1. Mr Locke hath observed, that when we perceive a number of qualities always existing together, we may gather from thence, that there is really some substance which produces these qualities. 2. The hypothesis of elementary fire is simple, and agreeable to the phenomena. 3. From some experiments made by Sir Isaac Newton it appears, that bodies acquire heat and cold in vacuo, until they become of the same temperature with the atmosphere; so that heat exists in the absence of all other matter, and is therefore a substance by itself.

"Our senses are no just measures of the degrees of heat and cold:—but the thermometer truly shews the increase or diminution of heat in the same body, while it preserves the same form: but this it will only do within certain limits. For when fluids are on the point of freezing, they contract irregularly, and expand in the same manner when brought near the boiling point. The thermometer, however, cannot shew the absolute quantity of heat contained in any body, because the beginning of the thermometrical scale by no means denotes the total absence of heat. For the same reason, it cannot even shew us the proportion between the quantities of heat contained in two different bodies; but Dr Black hath discovered a very ingenious and accurate method of shewing this last.

"It is universally known, that when two substances of different temperatures are mixed together, the

one loses and the other acquires heat, till they both appear of the same temperature. Let, therefore, any quantity of water be mixed with an equal bulk of any other substance, either hotter or colder than itself, and observe what happens on the mixture. If the one gains as much as the other loses, both of them contain the same quantity of heat, when in their natural state; but if the one gains more than the other loses, then their natural quantities of heat differ in the same proportion.

"If any body, heated beyond the common temperature of the air, is exposed to it, the heat flows out from it into the atmosphere, and disposes itself equally around, till the air becomes of the same temperature with itself. The same happens to bodies suspended in vacuo. Hence it is justly concluded, there exists between the particles of heat a repulsive power, by which they mutually recede from each other.

"Notwithstanding this repulsive power, however, the quantities of heat contained in different substances even of the same temperature, are found to be altogether different. This is sufficiently proved by some experiments made by Fahrenheit and Boerhaave, and still further confirmed by others made by the learned Dr Black. He took equal bulks of water and mercury; and having heated the mercury 50° above the other, mixed them together as quick as possible. The temperature of the mixture was 20 degrees above the original temperature of the water. Again, having made the water 50° hotter than the mercury, the temperature of the mixture was 30° above the original temperature of the mercury. Hence it appears that the quantity of heat in water is to that in mercury when both are of an equal temperature, as 3 to 2. By similar experiments he hath also determined the quantities of heat contained in a great many different bodies; so that now it appears that the quantity of heat is scarce ever the same in any two different bodies; and hence we may conclude, that terrestrial bodies have a power of attracting heat, and that this power is different in different substances.

"From these principles it evidently follows, that heat is distributed among bodies directly in proportion to their attracting powers, and inversely according to the repulsive power between the particles of heat themselves.—Such is the distribution of heat among bodies in the neighbourhood of each other; and which is called the equilibrium of heat, because the thermometer shews no difference of temperature among them. For, seeing the heat is distributed according to the attracting power of each, the thermometer having also a proper attractive power of its own, can shew no difference in the quantity of heat contained in each; for which reason, all bodies in the neighbourhood of each other are soon reduced to the same temperature. Hence we can neither assent to the opinion of Boerhaave, who supposed that heat was distributed among bodies in proportion to their bulks; nor to the hypotheses of others, who imagined that they were heated in proportion to their densities. For the thermometer shews only the quantity of heat going out of a body, not that which is really contained in it; and both the above-mentioned hypotheses are overturned by the experiments already recited.

"This equilibrium of heat may be broken while the quantity of heat in the different bodies remains

the same. For seeing the heat is distributed through all bodies directly in proportion to their attraction of it, and inversely in proportion to the repulsion of the igneous particles; if in any body the former is diminished, or the latter augmented, the fire will flow out of the body until the equilibrium is again restored, and then the heat is said to be generated. On the contrary, if the attraction of any body towards heat is augmented, or the repulsion between its particles diminished, then heat will flow into the body, and cold is said to be generated.—This is explained by an experiment of Dr Cullen's. A thermometer suspended in the receiver of an air-pump, descended some degrees while the air was exhausting, but quickly recovered the temperature of the external atmosphere; and when the air was again admitted, it ascended beyond the temperature of the external air. While the air was exhausting, the thermometer descended, because the air which remained being rarified, the repulsion between the particles of fire contained in it was diminished, and therefore the heat flowed out from the thermometer; but being allowed to remain for a short time in vacuo, it acquired the temperature of the external air, and the equilibrium was restored. When the external air was admitted, then that which had remained in the receiver, and acquired the temperature of the external air, being suddenly compressed, the repulsion between the igneous particles was increased, and the heat entering into the thermometer caused it rise. From other experiments also it appears, that the temperature of the air becomes hotter by condensation, and colder by rarefaction; the reason of which is manifest from the principles already laid down.

“Fluidity and evaporation are justly reckoned general effects of heat; for there is scarce any body which cannot be liquefied, and even carried off into vapour by an intense heat. The fluidity of water is universally known to depend on heat. Mercury likewise, by an intense cold, may be deprived of its fluidity*. Air is a kind of vapour which always becomes denser by a diminution of its heat; and it is not improbable, that by a very great diminution of its heat the air itself might become solid; and this conclusion is confirmed by the analogy of other vapours.

“These phenomena may be explained from the principles already laid down. For the particles of all solids are connected by the attraction of cohesion; of which, as well as of every other attraction, the nature is such, that if any new attraction is induced, the former is weakened. As much heat, therefore, may be added to a body as may increase its power of attracting heat to such a degree, that the attraction of cohesion will be totally dissolved, and the particles will easily slide over one another, in which case the body becomes fluid. If the fire is still kept up, that fluid becomes rarer, the heat communicates the repulsion between its own particles to those of the fluid, which is now raised in vapour. Again, this vapour by a diminution of the heat is converted into a fluid, and by a still further diminution remains perpetually contracted into the form of a solid.

“If indeed such is the power of bodies to attract heat, and such the nature of heat as has been already laid down, we might conclude a priori, that the force by which any substance attracts heat would be increa-

fed while it was melting, or going off in vapour; that is, that liquefaction or evaporation would produce cold; and on the other hand, that when vapour was reduced into a liquid, or any fluid congealed into a solid, its power of attracting heat would be diminished; and therefore heat, according to the common phrase, would be generated. For when, by the solution of the body, the attraction of cohesion is taken away, the attraction of the body for heat is increased. And when evaporation takes place, the body, and therefore the heat which is in it, becomes more rare; the consequence of which is, that the repulsive power between the particles of fire is diminished, the equilibrium is taken off, and the heat enters the vapour on all sides.

“The learned philosopher Dr Black was the first who demonstrated experimentally that heat entered bodies in great quantity while they liquefy or are converted into vapour; and that the same quantity flowed out of them again while they were condensing into fluids, or congealing into solids; and from this principle he hath explained a great number of phenomena, in which heat or cold were formerly said to be generated. The heat, as long as it is inherent in these bodies, he calls latent; because, while it enters the body, it does not change its sensible temperature; nor, after it has entered, does it affect the thermometer; and he was of opinion, that upon a certain quantity of latent heat the states of fluidity and vapour principally depended.

“But an ingenious physician (Dr Irvin, professor of chemistry at Glasgow,) having made many experiments in order to find out the quantity of heat contained in different bodies, according to Dr Black's method, hath observed, that the same body, under different forms, shews different dispositions with respect to heat; that ice, for instance, mixed with mercury 20 degrees colder than itself, lessened the cold of the mixture in a smaller degree, or imparted to it a smaller quantity of heat than water mixed with mercury 20 degrees colder than itself. Hence that learned gentleman hath concluded, that the great quantity of heat which is found to enter into bodies while they evaporate, or are reduced to a fluid state, is absorbed by them because of their change of form, and consequently of their disposition towards heat; and hence that the entrance of the heat is not to be accounted the cause, but the effect or consequence, of fluidity or vapour. This opinion indeed seems more probable, and agreeable to the principles already laid down, than the other. But we must carefully remember, that the thermometer can only measure that quantity of heat which flows out from bodies, not that which enters into them. If, therefore, a quantity of heat in vapour affects the thermometer less than an equal quantity in water, we are thence to conclude, that vapour has a stronger attraction for heat than water, and therefore the thermometer attracts a smaller quantity of heat from it than from water. When melting ice absorbs a great quantity of heat, which yet does not affect the thermometer; and when water converted into ice, throws out a great quantity of heat upon the thermometer or other bodies around it; these phenomena are easily explained from the greater attraction which water has for heat.

“The increase of the attraction which water has for

for heat, arises from the solution of its attraction of cohesion, as appears probable from analogy. But heat rushes into vapour, because it is distributed among bodies directly in proportion to their attractive powers, and inversely in proportion to the repulsive power between its own particles. When water is converted into vapour, its parts are removed to a distance from each other, and consequently the particles of heat. The repulsion between the latter, therefore, is diminished; the heat flows into the vapour, until the attraction between its particles with respect to the attracting power of the body becomes the same as before; whence we plainly see the reason why vapour absorbs the greater quantity of heat in proportion as it is more expanded.

"That heat accompanies the rays of the sun, is beyond all doubt; but whether the sun communicates heat to the rays of light, or whether they attract it from the air, is with me a matter of uncertainty. If heat is derived from the sun, how comes it to pass that the earth is no hotter after having received such an immense quantity of heat for so many ages? If the heat is communicated from the earth to the air, and is not again carried off from thence, why is the atmosphere so cold in its upper parts; or why has not the equilibrium of heat yet pervaded the whole atmosphere? If the rays of light have received heat from the sun himself, why does not an equal quantity of heat accompany the same quantity of light at all different distances from the earth?—These, and other objections, shew, that heat does not flow from the sun; and some of them plainly shew, that heat is attracted from the atmosphere; which thought we shall now prosecute a little farther.

"Heat is of such a nature, as I have already shewn, that, when accumulated, it diffuses itself all around, and joins with bodies in proportion to their attractive powers; but if the rays of light are collected into a focus in the air, and no more heat is added to the air, the quantity of heat in the collected rays is not increased in proportion to their own attraction, more than of the air in proportion to its attraction. But if they are in equilibrium as to their heat, before the collection by the focus, this shews, that heat has entered the rays of light from the atmosphere, seeing their temperature increases by condensation. But that the rays of the sun really do attract heat from the atmosphere, seems to be proved by the observations of De Luc; who hath observed, that the heat is very much, and very suddenly, decreased before sun-rising; which shews, that the rays of the sun, sweeping along at a small distance above the earth, abstracted the heat from that part of the atmosphere, which therefore flowed from the lower parts to supply the place of that which was taken away."

From this last paragraph it seems natural to conclude, that the sun, instead of being the source of heat, is really a source of cold; and the atmosphere the only reservoir of heat to us. But, on this supposition, it seems difficult to conceive how the heat of the atmosphere could remain the same in quantity, even for a single moment. The rays of the sun, it is said, cool the atmosphere, and carry off its heat; while they sweep along it at a little distance from the surface of the earth. But this they are perpetually doing. While the sun, for instance, is shining directly

upon Britain, his rays are cooling the atmosphere above America. But these rays which thus pass through the atmosphere without touching the earth's surface, fly into the most remote regions of space, and consequently must carry the heat of our atmosphere along with them, where it must be for ever lost to us. Thus the general quantity of heat would be perpetually diminishing; and in such a number of ages as have intervened from the beginning of the world, it must have been totally dissipated, or diffused through spaces to which our earth and its atmosphere bear not the proportion of 1 to 10,000,000,000.

His explanation of Dr Black's principle of latent heat is, perhaps, equally liable to objection. We shall for a moment allow, that the absorption of heat is the consequence, and not the cause, of fluidity and vapour: but what then is the cause? It seems undeniable, that the cause is a quantity of heat, greater than what the body is naturally fitted to contain, forced upon it ab extra: but this heat, the moment it enters the body, is to appearance lost and annihilated, while the body changes its form. How is it possible, then, to assign another cause for the change of form in the body, than its absorption of the quantity of heat which was forced upon it ab extra?—To say that the body now attracts heat from the atmosphere in greater quantity than it did before, and which enters it copiously, cannot be admitted. The body did not originally attract the heat; it was plainly forced upon it; a quantity of heat entered it, and changed its form. This quantity therefore remains within the body, and neither attracts nor repels that which is in the atmosphere. The body can attract no more, unless that which has already entered was to be thrown out; and therefore the coldness of vapour cannot be owing to its attraction of heat from the atmosphere: neither is it owing to its attracting heat more than water does; for vapour may be heated in such a manner, that it will part with heat as readily as water, or any other substance whatever.—Nor does the experiment adduced in favour of this doctrine seem conclusive. Ice communicates less heat to mercury 20 degrees colder than itself, than water does; but for this two reasons may be assigned. 1. When bodies are intensely heated, they part with their heat very readily, and in great quantity; but as the heat decreases, it also flies off, or is communicated to surrounding bodies more slowly and with more difficulty than before. Water always contains a greater quantity of heat than ice, and therefore ought to part with an equal quantity of heat more readily and easily than ice does. 2. When water is cooled to 32 degrees, it freezes; when just above 32 degrees, it becomes fluid; and, with every degree of heat superior to that, evaporates. If water heated to 212 degrees, or near it, is placed within the receiver of an air-pump, and the air exhausted, a great quantity of steam issues from it, and the heat of the water suddenly decreases to 98 or 100 degrees. Hence it is plain, that 112 degrees of the heat of boiling water is contained, not in the water itself, but in the steam detained among its particles by the pressure of the atmosphere. Had this water, therefore, been mixed with mercury much colder than itself, a quantity of the steam would have been condensed by the coldness of the metal. But steam cannot be condensed without

without giving out, not only its sensible heat, but that which is latent also; and hence we may easily see why water communicates degrees of heat so much greater in proportion than ice does.

Lastly, The principle on which the whole doctrine is founded, does not seem to agree either with the phenomena of nature, or with the conclusions which must be drawn from itself. It is said, that fire tends to diffuse itself equally on all bodies in the neighbourhood of one another. But does this hold in fact? Heat has a disposition to ascend; and that not only in the atmosphere, but in vacuo, and through solid bodies. If a bar of iron, red-hot at one end, is set to cool with its red part undermost, the heat will ascend farther and quicker through it, than it will descend if the iron is placed the contrary way. The argument, therefore, for the repulsive power between the particles of heat does not hold, nor that for the attraction between the particles of heat and those of other matter. Though different bodies contain different quantities of heat, this difference may be occasioned otherwise than by attraction. We have already seen, that a certain quantity of heat absorbed by water in the state of ice, is the cause of its fluidity. The same is the case with mercury; but the latter can remain fluid with much less heat than water can. It must, however, be admitted, that the substances we call water and mercury are only so in consequence of the action of heat. Thus, water deprived of a certain quantity of heat, is no longer water, but a kind of glass; and mercury is no longer mercury, but a solid metal. Now, let us suppose, that ice at 32 degrees is applied to a quantity of mercury at 32; we know that the ice is only water deprived of a quantity of its natural heat, and of consequence it ought to have a violent attraction for that quantity which has been forced from it. The mercury has a great deal of heat to spare, yet the ice attracts none from it; neither will the fluid water attract heat from mercury of the same temperature, though the one is said to attract it more strongly than the other. That water should be heated by mercury hotter than itself, is not wonderful; nor can we account this an effect of attraction, because the superfluous quantity of heat would not remain in the mercury, though the water was not applied to it.

From these, and a multitude of other considerations, we would conclude, that heat itself, and not the attractive and repulsive power supposed by Dr Cleghorn and others, is the active principle which has so great a share in the operations of nature. The principles on which the actions of heat depend, may perhaps be explained in a less exceptionable manner, from the following propositions.

1. It is in all cases observed, That when light proceeds in considerable quantity from a point, diverging as the radii of a circle from its centre, there a considerable degree of heat is found to exist, if an opaque body, having no great reflective power, is brought near that point.

2. This action of the light, therefore, may be accounted the ultimate cause of heat, without having recourse to any farther suppositions; because nothing else besides this action is evident to our senses.

3. If the point from which the rays are emitted is placed in a transparent medium, such as air or water,

that medium, without the presence of an opaque body, will not be heated.

4. Another cause of heat, therefore, is the resistance of the parts of that body on which the light falls, to the action mentioned in Prop. 1. Where this resistance is weak, as in the cases just mentioned, the heat is either nothing, or very little.

5. If a body capable of reflecting light very copiously is brought near the lucid point, it will not be heated *.

6. A penetration of the light, therefore, into the article substance of the body, and likewise a considerable degree of resistance on the part of that body to the action of the light, are the requisites to produce heat. * See BURNING-Glass.

7. Those bodies ought to conceive the greatest degrees of heat, into whose substance the light can best penetrate, i. e. which have the least reflective power, and which most strongly resist its action; which is evidently the case with black and solid substances.

8. By heat all bodies are expanded in their dimensions every way, and that in proportion to their bulk and the quantity of heat communicated to them.

9. This expansion takes place not only by an addition of sensible heat, but likewise of that which is latent. Of this last we have a remarkable instance in the case of snow mixed with spirit of nitre. The spirit of nitre contains a certain quantity of latent heat, which cannot be separated from it without effecting a change on the spirit itself; so that, if deprived of this heat, it would no longer be spirit of nitre.— Besides this, it contains a quantity of sensible heat, of a great part of which it may be deprived, and yet retain its characteristic properties as nitrous acid. When it is poured upon snow, the latter is immediately melted by the action of the latent heat in the acid. The snow cannot be melted or converted into water, without imbibing a quantity of latent heat, which it receives immediately from the acid which melts it. But the acid cannot part with this heat without decomposition; to prevent which, its sensible heat occupies the place of that which has entered the snow and liquefied it. The mixture then becomes exceedingly cold, and the heat forces into it from all the bodies in the neighbourhood; so that, by the time it has recovered that quantity of sensible heat which was lost, or arrived at the temperature of the atmosphere around it, it will contain a considerably larger quantity of heat than it originally did, and is therefore observed to be expanded in bulk. Another instance of this expansive power of latent heat is in the case of steam, which always occupies a much larger space than the substance from which it was produced; and this whether its temperature is greater or less than the surrounding atmosphere.

10. The difference between latent and sensible heat, then, as far as we can perceive, is, that the expansive power of the first is directed only against the particles of which the body is composed; but that of the second is directed also against other bodies. Neither doth there seem to be any difference at all between them farther than in quantity. If water, for instance, hath but a small quantity of heat, its parts are brought near each other, it contracts in bulk, and feels cold. Still, however, some part of the heat is detained among the aqueous particles, which prevents the fluid from

from congealing into a solid mass. But, by a continuation of the contracting power of the cold, the particles of water are at last brought so near each other that the internal or latent heat is forced out. By this discharge a quantity of air is also produced, the water is congealed, and the ice occupies a greater space than the water did; but then it is full of air-bubbles, which are evidently the cause of its expansion. The heat then becomes sensible, or, as it were, lies on the outside of the matter; and consequently is easily dissipated into the air, or communicated to other bodies. Another way in which the latent heat may be extricated is by a constant addition of sensible heat. In this case the body is first raised into vapour, which for some time carries off the redundant quantity of heat. But as the quantity of this heat is continually increased, the texture of the vapour itself is at last totally destroyed. It becomes too much expanded to contain the heat, which is therefore violently thrown out on all sides into the atmosphere, and the body is said to burn, or be on fire. See FLAME and IGNITION.

11. Hence it follows, that those bodies which have the least share of latent heat, appear to have the greatest quantity of sensible heat; but this is only in appearance, for the great quantity they seem to contain is owing really to their inability to contain it. Thus, if we can suppose a substance capable of transmitting heat through it as fast as it received it; if such a substance was set over a fire, it would be as hot as the fire itself, and yet the moment it was taken off, it would be perfectly cool, on account of its incapacity to detain the heat among the particles of which it was composed.

12. The heat, therefore, in all bodies, consists in a certain violent action of the elementary fire within them tending from a centre to a circumference, and thus making an effort to separate the particles of the body from each other, and thereby to change its form or mode of existence. When this change is effected, bodies are said to be dissipated in vapour, calcined, vitrified, or burnt, according to their different natures.

13. Inflammable bodies are such as are easily raised in vapours; that is, the fire easily penetrates their parts, and combines with them in such quantity, that, becoming exceedingly light, they are carried up by the atmosphere. Every succeeding addition of heat to the body increases also the quantity of latent heat in the vapour, till at last, being unable to resist its action, the heat breaks out all at once, the vapour is converted into flame, and is totally decomposed. See the article FLAME, and Prop. 10.

14. Uninflammable bodies are those which have their parts more firmly connected, or otherwise disposed in such a manner, that the particles of heat cannot easily combine with them or raise them into vapour.

15. Heat therefore being only a certain mode of the action of elementary fire, it follows, that the capacity of a body for containing it, is only a certain constitution of the body itself, or a disposition of its parts which can allow the elementary fire contained in it to exert its expansive power upon them without being dissipated on other bodies. Those substances which allow the expansive power of the fire to operate on their own particles are said to contain a great deal of

heat; but those which throw it away from themselves upon other bodies, though they feel very hot, yet philosophically speaking they contain very little.

16. What is called the quantity of heat contained in any substance, if we would speak with the strictest propriety, is only the apparent force of its action either upon the parts of the body itself, or upon other bodies in its neighbourhood. The expansive force of the elementary fire contained in any body upon the parts of that body is the quantity of latent heat contained in it; and the expansive force of the fire exerted upon other bodies which touch or come near it, is the quantity of sensible heat it contains.

17. If what we call heat consists only in a certain action of that fluid called elementary fire, namely, its expansion, or acting from a centre to a circumference, it follows, that if the same fluid act in a manner directly opposite to the former, or press upon the particles of a body as from a circumference to a centre, it will then produce effects directly opposite to those of heat, i. e. it will then be absolute cold, and produce all the effects already attributed to COLD. See that article.

18. If heat and cold then are only two different modifications of the same fluid, it follows, that if a hot body and a cold one are suddenly brought near each other, the heat of the one ought to drive before it a part of the cold contained in the other, i. e. the two portions of elementary fire acting in two opposite ways, ought in some measure to operate upon one another as any two different bodies would when driven against each other. When a hot and a cold body therefore are brought near each other, that part of the cold body farthest from the hot one ought to become colder than before, and that part of the hot body farthest from the cold one ought to become hotter than before.

19. For the same reason, the greatest degree of cold in any body ought to be no obstacle, or at least very little, to its conceiving heat, when put in a proper situation. Cold air, cold fuel, &c. ought to become as intensely heated, and very nearly as soon, as that which is hotter.

The two last propositions are of great importance. When the first of them is thoroughly established, it will confirm beyond a doubt, that cold is a positive as well as heat; and that each of them has a separate and distinct power, of which the action of its antagonist is the only proper limit; i. e. that heat can only limit the power of cold, and vice versa. A strong confirmation of this proposition is the experiment related by M. Geoffroy; an account of which is given under the article COLD, n^o 5. Another, but not so well authenticated, is related under the article CONGELATION, n^o 11.—De Luc's observation also, mentioned by Dr Cleghorn affords a pretty strong proof of it; for if the lower parts of the atmosphere are cooled by the passage of the sun's rays at some distance above, and it hath been already shewn that they do not attract the heat from the lower parts, it follows, that they must expel part of the cold from the upper regions.—The other proposition, when fully established, will prove, that heat and cold are really convertible into one another; which indeed seems not improbable, as we see that fires will burn with the greatest fierceness during the time of intense frosts, when the coldest air is admitted

mitted to them; and even in those dismal regions of Siberia, when the intense cold of the atmosphere is sufficient to congel quicksilver, it cannot be doubted that fires will burn as well as in this country; which could not happen if heat was a fluid per se, and capable of being carried off, or absolutely diminished in quantity, either in any part of the atmosphere itself, or in such terrestrial bodies as are used for fuel.

This theory of heat, if found admissible by philosophers, would introduce a greater simplicity into natural philosophy, and explain the phenomena of nature in a manner much more easy and less liable to objection, than hath yet been done: we would then see the phenomena of attraction, repulsion, electricity, fire, light, ignition, &c. &c. to be only as many different effects of one ethereal fluid, which the Deity hath appointed to be the first of all secondary causes, and to alter, modify, compound, and separate, the different parts of terrestrial substances, as we daily see done by the different natural agents.

Heat of Burning Bodies. See IGNITION.

Heat of Chemical Mixtures. This is a phenomenon necessarily resulting from the change of form produced in the different substances which are mixed together; and the manner in which it happens may be easily understood from the example of oil of vitriol and water. If equal quantities of concentrated vitriolic acid and water are mixed together, a very great degree of heat immediately takes place; inasmuch, that if the vessel which contains the mixture is made of glass, it will probably break; and after it is cold, the mixture will be found to have shrunk in its dimensions, or will occupy less space than the bulk of the water and acid taken separately. In this case we know that the water, while in its fluid state, hath as much latent heat as it can contain; i. e. the elementary fire within it expands, or separates its parts from each other, as much as is consistent with the constitution of the body. If any more is added, it cannot be absorbed, or direct its force upon the particles of the water without raising them in vapour: of consequence, part of this additional expansive power will be employed in the formation of vapour, and the rest will be discharged upon the neighbouring bodies; i. e. will be converted into sensible heat. The vitriolic acid, in its concentrated state, contains a great quantity of latent heat which is necessary to preserve its fluidity. But when it is mixed with the fluid water, the latent heat contained in the latter is abundantly sufficient for both: of consequence, the great expansive power in the oil of vitriol itself becomes now totally useless, and therefore exerts its force upon the neighbouring bodies; and when the mixture returns to the original temperature of the oil of vitriol and water, it shews a loss of substance by its diminution in bulk. This may serve to explain all cases in chemistry where heat or cold is produced: and it will generally be found, that where bodies, by being mixed together, produce heat, they shrink in their dimensions; but when they produce cold, they are enlarged.

Heat of Climates. See HEAT, n^o 2; and AMERICA, n^o 3...24.

Methods of Measuring HEAT. See THERMOMETER.

Expansion of Metals by HEAT. See PYROMETER.

Degrees of HEAT which Animals are capable of bear-

ing. The ancients were of opinion that all countries lying within the tropics were uninhabitable by reason of their heat; but time has discovered their mistake, and it is now found that no part of the world is too hot for mankind to live in. The learned professor Boerhaave, in his chemistry, relates certain experiments made with great accuracy by the celebrated Fahrenheit, and others, at his desire, on this subject, in a sugar-baker's office; where the heat, at the time of making the experiments, was up to 146 degrees of Fahrenheit's thermometer. A sparrow, subjected to air thus heated, died, after breathing very laboriously, in less than seven minutes. A cat resisted this great heat somewhat above a quarter of an hour; and a dog about 28 minutes, discharging, before his death, a considerable quantity of a ruddy-coloured foam, and exhaled a stench so peculiarly offensive, as to throw one of the assistants into a fainting fit. This dissolution of the humours, or great change from a natural state, the professor attributes not to the heat of the stove alone, which would not have produced any such effect on the flesh of a dead animal; but likewise to the vital motion, by which a still greater degree of heat, he supposes, was produced in the fluids circulating through the lungs, in consequence of which the oils, salts, and spirits of the animal became so highly exalted.

Messieurs Du-Hamel and Tillet having been sent into the province of Augomois, in the years 1760 and 1761, with a view of endeavouring to destroy an insect which consumed the grain of that province, effected the same in the manner related in the Memoirs for 1761, by exposing the affected corn, with the insects included in it, in an oven, where the heat was sufficient to kill them without injuring the grain. This operation was performed at Rochefoucault, in a large public oven, where, for economical views, their first step was to assure themselves of the heat remaining in it, on the day after bread had been baked in it. This they did, by conveying in a thermometer on the end of a shovel, which, on its being withdrawn, indicated a degree of heat considerably above that of boiling water: but M. Tillet, convinced that the thermometer had fallen several degrees in drawing to the mouth of the oven, and appearing under some embarrassment on that head, a girl, one of the attendants on the oven, offered to enter, and mark with a pencil the height at which the thermometer stood within the oven. The girl smiled on M. Tillet's appearing to hesitate at this strange proposition; and entering the oven, with a pencil given her for that purpose, marked the thermometer, after staying two or three minutes, standing at 100 degrees of Reaumur's scale, or, to make use of a scale better known in this country, at near 260 degrees of Fahrenheit's. M. Tillet began to express an anxiety for the welfare of his female assistant, and to press her return. This female salamander, however, assuring him that she felt no inconvenience from her situation, remained there 10 minutes longer; that is, near the time when Boerhaave's cat parted with her nine lives, under a much less degree of heat; when the thermometer standing at 288 degrees, or 76 degrees above that of boiling water, she came out of the oven, her complexion indeed considerably heightened, but her respiration by

Heat. means quick or laborious. After M. Tillet's return to Paris, these experiments were repeated by M. Marantin, commissaire de guerre, at Rochefoucault, an intelligent and accurate observer, on a second girl belonging to the oven; who remained in it, without much inconvenience, under the same degree of heat, as long as her predecessor; and even breathed in air heated to about 325 degrees, for the space of five minutes.

M. Tillet endeavoured to clear up the very apparent contrariety between these experiments and those made under the direction of Boerhaave, by subjecting various animals, under different circumstances, to great degrees of heat. From his experiments, in some of which the animals were swaddled with clothes, and were thereby enabled to resist for a much longer time the effects of the extraordinary heat, he infers, that the heat of the air received into the lungs was not, as was supposed by Boerhaave, the only or principal cause of the anxiety, laborious breathing, and death, of the animals on whom his experiments were made; but that the hot air, which had free and immediate access to every part of the surface of their bodies, penetrated the substance on all sides, and brought on a fever, from whence proceeded all the symptoms: on the contrary, the girls at Rochefoucault, having their bodies in great measure protected from this action by their clothes, were enabled to breathe the air, thus violently heated, for a long time without great inconvenience. In fact, we should think too, that the bulk of their bodies, though not thought of much consequence by M. Tillet, appears to have contributed not a little to their security. In common respiration, the blood, in its passage through the lungs, is cooled by being brought into contact with the external inspired air: In the present experiments, on the contrary, the vessels and vessels of the lungs receiving at each inspiration an air heated to 300 degrees, must have been continually cooled and refreshed, as well as the subcutaneous vessels, by the successive arrival of the whole mass of blood contained in the interior parts of the body, whose heat might be supposed at the beginning of the experiment not to exceed 100 degrees. Not to mention, that M. Tillet's two girls may not possibly have been subjected to so great a degree of heat as that indicated by the thermometer; which appears to us to have always remained on the shovel, in contact with the earth.

These experiments soon excited other philosophers to make similar ones, of which some very remarkable ones are those of Dr Dobson at Liverpool, who gives the following account of them in the Philosophical Transactions, vol. lxv.

"I. The sweating-room of our public hospital at Liverpool, which is nearly a cube of nine feet, lighted from the top, was heated till the quicksilver stood at 224° on Fahrenheit's scale, nor would the tube of the thermometer indeed admit the heat to be raised higher. The thermometer was suspended by a string fixed to the wooden frame of the fly-light, and hung down about the centre of the room. Myself and several o-

thers were at this time inclosed in the stove, without experiencing any oppressive or painful sensation of heat proportioned to the degree pointed out by the thermometer. Every metallic about us soon became very hot.

"II. My friend Mr Park, an ingenious surgeon of this place, went into the stove heated to 202°. After ten minutes, I found the pulse quickened to 120. And to determine the increase of the animal-heat, another thermometer was handed to him, in which the quicksilver already stood at 98°; but it rose only to 99½, whether the bulb of the thermometer was inclosed in the palms of the hands, or received in the mouth (A). The natural state of this gentleman's pulse is about 65.

"III. Another gentleman went through the same experiment in the same circumstances, and with the same effects.

"IV. One of the porters to the hospital, a healthy young man, and the pulse 75, was inclosed in the stove when the quicksilver stood at 210°; and he remained there, with little inconvenience, for 20 minutes. The pulse, now 164, and the animal-heat, determined by another thermometer as in the former experiments, was 101½.

"V. A young gentleman of a delicate and irritable habit, whose natural pulse is about 80, remained in the stove ten minutes when heated to 224°. The pulse rose to 145, and the animal-heat to 102°. This gentleman, who had been frequently in the stove during the course of the day, found himself feeble, and disposed to break out into sweats for 24 hours after the experiment.

"VI. Two small tin vessels, containing each the white of an egg, were put into the stove heated to 224°. One of them was placed on a wooden seat near the wall, and the other suspended by a string about the middle of the stove. After ten minutes, they began to coagulate; but the coagulation was sensibly quicker and firmer in that which was suspended, than in that which was placed on the wooden seat. The progress of the coagulation was as follows: it was first formed on the sides, and gradually extended itself; the whole of the bottom was next coagulated; and last of all, the middle part of the top.

"VII. Part of the shell of an egg was peeled away, leaving only the film which surrounds the white; and part of the white being drawn out, the film sunk so as to form a little cup. This cup was filled with some of the albumen ovi, which was consequently detached as much as possible from every thing but the contact of the air and of the film which formed the cup. The lower part of the egg stood upon some light tow in a common gallipot, and was placed on the wooden seat in the stove. The quicksilver in the thermometer still continued at 224°. After remaining in the stove for an hour, the lower part of the egg, which was covered with the shell, was firmly coagulated; but that which was in the little cup, was fluid and transparent. At the end of another hour it was still fluid, except on the edges where it was thinnest; and here it was still transparent; a sufficient proof that it

Heat. it was dried, not coagulated.

"VIII. A piece of bees-wax, placed in the same situation with the albumen ovi of the preceding experiment, and exposed to the same degree of heat in the stove, began to melt in five minutes: another piece suspended by a string, and a third piece put into the tin vessel and suspended, began likewise to liquefy in five minutes."

Even these experiments, though more accurate than the former, do not shew the utmost degrees of heat which the human body is capable of enduring. Some others, still more remarkable, (as in them the body was exposed to the heat without clothes), by Drs Fordyce and Blagden, are also recorded in the Philosophical Transactions. They were made in rooms heated by flues in the floor, and by pouring upon it boiling water. There was no chimney in them, nor any vent for the air, excepting through crevices at the door. In the first room were placed three thermometers, one in the hottest part of it, another in the coolest part, and a third on the table, to be used occasionally in the course of the experiment. Of these experiments, the two following may be taken as a specimen.

"About three hours after breakfast, Dr Fordyce having taken off all his clothes, except his shirt, and being furnished with wooden shoes tied on with lill, went into one of the rooms, where he staid five minutes in a heat of 90°, and begun to sweat gently. He then entered another room, and stood in a part of it heated to 110°. In about half a minute his shirt became so wet, that he was obliged to throw it aside, and then the water poured down in streams over his whole body. Having remained in this heat for ten minutes, he removed to a part of the room heated 120°; and after staying there 20 minutes, found that the thermometer placed under his tongue, and held in his hand, stood just at 100°, and that his urine was of the same temperature. His pulse had gradually risen to 145 pulsations in a minute. The external circulation was greatly increased, the veins had become very large, and an universal redness had diffused itself all over the body, attended with a strong feeling of heat; his respiration, however, was little affected. He concluded this experiment by plunging in water heated to 100°; and after being wiped dry, was carried home in a chair; but the circulation did not subside for two hours."

"Dr Blagden took off his coat, waistcoat, and shirt, and went into one of the rooms, as soon as the thermometer had indicated a degree of heat above that of boiling water. The first impression of this hot air upon his body was exceedingly disagreeable, but in a few minutes all his uneasiness was removed by the breaking out of a sweat. At the end of 12 minutes he left the room very much fatigued, but no otherwise disordered. His pulse beat 136 in a minute, and the thermometer had risen to 220 degrees."

In others of these experiments it was found, that a heat even of 260° Fahrenheit's thermometer could be submitted to with tolerable ease. But it must be observed, that in these great heats every piece of metal they carried about with them become intolerably hot. Small quantities of water placed in metalline vessels quickly boiled; but in a common earthen ves-

sel it required an hour and an half to arrive at a temperature of 140°, nor could it ever be brought near the boiling point. Neither durst the people, who with impunity breathed the air of this very hot room at 264 degrees, bear to put their fingers into the boiling water which indicated only a heat of 212°. So far from this, they could not bear the touch of quicksilver heated only to 120°, and could but just bear spirit of wine at 130°.

Animal-Heat. Of this there are various degrees: some animals preserving a heat of 100° or more in all the different temperatures of the atmosphere; others keep only a few degrees warmer than the medium which surrounds them; and in some of the more imperfect animals, the heat is scarcely one degree above the air or water in which they live.

The phenomenon of animal-heat hath, from the earliest ages, been the subject of philosophical discussion; and, like most other subjects of this nature, its cause is not yet ascertained. The best treatises that have appeared on the subject are those of Dr Dugald Leslie, published in 1778; and Mr Adair Crawford, in 1779. From the first of these performances, the following account of the different opinions on this subject is extracted.

"The ancients possessed not the requisites for minutely investigating the science of nature; and, prone to superstition, attributed every phenomenon which excluded their investigation, to the influence of a supernatural power. Hippocrates, the father and founder of medicine, accounted animal-heat a mystery, and bestowed on it many attributes of the deity. In treating of that subject, he says in express terms, "what we call heat, appears to me to be something immortal, which understands, sees, hears, and knows every thing present and to come."—Aristotle seems to have considered the subject particularly, but nothing is to be met with in his works that can be said to throw light upon it.—Galen tells us that the dispute between the philosophers and physicians of his time was, "whether animal-heat depended on the motion of the heart and arteries; or whether, as the motion of the heart and arteries was innate, the heat was not also innate." Both these opinions, however, he rejects; and attempts a solution of the question on his favourite system, namely, the peripatetic philosophy: but his leading principles being erroneous, his deductions are of course inadmissible.

"To enter into a minute detail of all the opinions of the moderns on the cause of animal-heat, would far exceed our limits. Most of them, however, may be referred to one or other of the three general causes of heat, viz. mixture, fermentation, and mechanical means, each of which we shall particularly consider."

"1. Chemical mixture. When chemical philosophy first came into vogue, and prevailed in the theory as well as practice of medicine, almost every operation in the animal machine was said to be the effect of fermentation or mixture. From observing, that on the mixing of certain bodies far below the temperature of the human body, a degree of heat sometimes rising to actual inflammation was produced; they, without further investigation, pronounced mixture the sole cause of animal heat. Various, however, were the opinions,

not only respecting the place where the mixture happened, but also concerning the nature of the fluids of which it consisted. Van Helmont, Sylvius, and several others, supposed that the mixture took place in the intestinal tube; and ascribed it to an effervescence between the pancreatic juice and the bile. Others discovered acids in one place, and alkalies in another; but the general opinion for near two centuries was, That acident fluids taken in, meeting with others of an alkaline nature already prepared in the body, gave rise to the degree of heat peculiar to animals. But those who are in the least acquainted with the laws of the animal economy, need not be told that these opinions are mere conjectures, founded on facts gratuitously assumed. No experiments have shewn either an acidity or alkalinity in the bile that is sufficient to unite with the other animal juices, and generate the heat of animals. But though we should admit the supposition in its full extent, still it would by no means be sufficient to account for the stability of animal heat in different climates and seasons; its equality all over the body when in health; its partial increase in topical inflammations; or hardly indeed for any one phenomenon attending its production.

" Since, then, it appears that the fluids supposed to be mixed, the place in which the mixture is made, and every other circumstance relating to it, are neither ascertained nor seconded by analogy, none will, we presume, hesitate to reject every hypothesis of the cause of animal-heat founded on the effects of mixture.

" 2. Fermentation. When a more accurate and extensive knowledge of the various operations of nature had convinced physiologists of the absurdity of explaining the vital functions of animals, and the several changes which take place in the living body by the effects of chemical mixture, fermentation was substituted in its stead. All had observed, that fermentation was generally accompanied by heat; and few were ignorant, that that identical process, or one extremely similar to it, was constantly going forward in living animals; and it was not without some appearance of truth, that physiologists attributed animal-heat to that cause.

" Formerly there were various modifications of this opinion; but of late it has been chiefly confined to one species of fermentation, viz. the putrefactive, which indeed is more consentaneous to experience and found philosophy. For although animal-substances are either directly or indirectly produced from vegetables, as all animals live on vegetables, or on animals that have lived on them; and though they may be ultimately resolved into the same principles; yet they are certainly combined in a different manner: for they constitute compounds, the natures of which are essentially different; and of the three stages of fermentation, the vinous, acetous, and putrid, the last is the only one to which they shew a tendency. Milk indeed tends to the acetous, and even to the vinous fermentation; but as it can hardly be considered as perfectly animalized, it ought not to be considered as an exception to the general position. And though it be readily admitted, that animal matter is extremely apt to putrefy, and that even in the living body there is a tendency to that process; yet it may be shewn, that the degree to which it

takes place can have little or no share in generating the heat of animals. In the first place, the effect of any degree of putrefaction in producing heat, is to this day so ill ascertained, that, with many ingenious philosophers it is altogether problematical, whether or not animal substances, during the putrefactive process, do ever generate heat. Neither M. Beaume nor Dr Pearson, who made several accurate experiments with a view to ascertain this point, could, by the assistance of the most sensible thermometers, discover the least difference betwixt the temperature of the putrefying mixtures, and the surrounding medium; and were the putrefaction of animal-substances really attended with the generation of heat, we might expect to find it greater in proportion to the bulk of the putrefying mass. This, however, is not the case; for it has often been found, that the largest masses of animal matter, such as the carcass of a large whale, laid out and exposed to the air in such a putrid condition as to affect all the neighbourhood with an intolerable stench, did not to the persons handling it feel sensibly hotter than the circumambient air. But what at once overturns every thing that can be advanced in favour of the generation of animal-heat on the principles of putrefaction is, that heat is far more considerable in a living than in a dead body; and no rational physiologist will deny, that the putrid fermentation is going forward more rapidly in the latter than in the former.

" 3. The mechanical generation of heat. This opinion first took its rise from an observation, that animal-heat generally keeps pace with the state of the circulation: while the action of the heart and arteries continues unimpaired, a high degree of animal-heat is produced; but when that action becomes more languid, the heat of the animal is diminished also. This, till very lately, was the favourite opinion of physicians, and was introduced immediately after Harvey had discovered the circulation of the blood, and indeed seems to be supported by many striking facts. Physiologists looked upon it as a matter almost capable of mathematical demonstration; yet they could not agree whether the heat of animals is occasioned by the friction of the blood against the vessels which contain it, or by the internal friction and agitation of the particles among one another. Various hypotheses accordingly were framed, and many ingenious arguments brought in support of them: but all suppositions of the mechanical kind are overthrown by some thermometrical observations of De Haen and others, from which it appeared, that the heat of the body was sometimes greater than is usual with healthy people, at the time the person was just expiring, when the action of the vessels was very weak; nay, even after he was dead, when it had entirely ceased. The abovementioned physician relates two very remarkable cases of this kind. In the one, he found that the temperature of his patient, which during the course of an inflammatory fever had never exceeded 103 degrees, at the time he expired, and for two minutes after, stood at 106. From the other it appeared, that the heat of a person who was dying of a lingering distemper, rose in the last agony from 100 to 101, and continued there stationary for two hours; and, even at the expiration of 15 hours, had only fallen to 85°, though the surrounding medium did not exceed 60°. The examples also of those who

are suffocated by fixed air, entirely overturn not only the mechanical system, but almost every other which hath yet appeared on the subject. [See the article BLOOD, n° 31.]

"One or other of the abovementioned hypotheses continued to be adopted by physicians, till Dr Cullen attempted a solution on a new set of principles; but, attentive to the diffidence with which novel opinions ought to be broached, he delivered his as little more than a mere conjecture. 'May it not (says he) be supposed, That there is some circumstance in the vital principle of animals, which is in common to those of the same class, and of like economy; and which determines the effect of motion upon the vital principle to be the same, though the motion acting upon it may be in different circumstances?'—The doctor was driven to this supposition from the difficulty he found in explaining how so many animals of a different age, size, and temperament, should possess very nearly the same degree of heat; and in which it is impossible to shew, that the motion of the blood in all its circumstances, is exactly the same; or that in the different animals in which the degree of heat is considerably different, the motion of the circulating mass is, in each, correspondent to the difference of temperature. But, granting that the degree of heat does not always obtain in an exact ratio with the motion of the blood, and that this is an insuperable objection to its mechanical generation, yet there appear no plausible grounds for supposing that the effect of motion may be the same, while the motion acting upon it is in different circumstances. By this Dr Cullen means, That the different temperature of different animals is owing to a difference of the vital principle, inasmuch that the velocity of the blood may be the same in a frog as in a man; and yet, in consequence of the different vital principle, the heat produced may be different. The facts upon which he seems to lay the greatest stress are, That neither where the surrounding medium considerably surpasses the temperature of the living body, nor where it is far below it, is there any sensible change in the heat of animals. These, and some similar facts, in appearance countenance his hypothesis; yet we have no solid reason for imagining the principle of life to be different in different animals. And how are we to conceive, that the same degree of motion should in one class of animals always produce a certain degree of heat, and in another class as regularly a different one? A proposition of such a nature should, no doubt, require the most obvious facts and conclusive arguments to establish it; but, in the present instance, we do not perceive any probable reason, even from analogy. Besides, to say that the principle of life can generate heat or cold, independent of chemical or mechanical means, is contrary to experience, and seems in itself absurd.

"In the 66th volume of the Philosophical Transactions, Dr Hunter, after reciting some experiments concerning animal-heat, asserts, That certain animals entirely destitute of nerves, are endowed with a power of generating their own heat; and this he brings as an argumentum crucis against those who account the nervous system the seat of animal-heat. If this is really a fact, it must, no doubt, have all the weight he ascribes to it; but it is plain that no stress can be

laid upon it, unless it was better ascertained, which it is evident it never can be. For though we can positively assert that nerves exist where we see them, yet we cannot affirm with equal certainty that they do not also exist where we are not able to discover them. For all anatomists allow, that there are thousands of nervous filaments so finely interwoven into the composition of the more perfect animals of every size, that they elude not only the knife and naked eye, but even the best optical instruments hitherto invented. Since then we admit the presence of nerves in one tribe of animals, though we can only perceive them in their effects; what solid reason have we to deny them in another, in which we have the very same evidence, viz. certain indications of sense and motion?

"Another theory, and perhaps the best supported which hath yet appeared on the subject, is that of Dr Black's. That excellent chemist having observed, That not only breathing animals are of all others the warmest, but also that there subsists such a close and striking connexion between the state of respiration and the degree of heat in animals, that they appear to be in an exact proportion to one another, was led to believe, that animal-heat depends on the state of respiration; that it is all generated in the lungs by the action of the air upon the principle of inflammability, in a manner little dissimilar to what he supposed to occur in actual inflammation; and that it is thence diffused by means of the circulation over the rest of the vital system.

"This opinion is supported by many forcible arguments. 1. It is pretty generally known to naturalists, that a quantity of mephitic phlogisticated air is constantly exhalating from the lungs of living animals.—Since, therefore, atmospheric air, by passing through the lungs, acquires the very same properties as by passing through burning fuel, or by being exposed to any other process of phlogistication, it is obvious, that the change which the common air undergoes in both cases, must be attributed to one and the same cause, viz. its combination with phlogiston. 2. It has likewise been urged in favour of the same hypothesis, That the celerity with which the principle of inflammability is separated in respiration, is very closely connected with the degree of heat peculiar to each animal. Thus, man, birds, and quadrupeds, vitiate air very fast; serpents, and all the amphibious kind, very slowly; and the latter are of a temperature inferior to the former, and breathe less frequently. 3. The most cogent arguments that have been brought in support of this opinion are, That no heat is generated till the function of respiration is established; and that the foetus in utero derives all its heat from the mother."

Upon this theory our author makes the following observations, which we shall give in his own words.

"These arguments may, perhaps, on a superficial view of the question, appear conclusive; but a sounder reasoner, who shall coolly and impartially weigh every circumstance, will, I am confident, allow that they only afford a very ambiguous and imperfect evidence of the doctrine they are meant to establish: and the subsequent animadversions on Dr Black's theory at large, will, it is hoped, suffice to shew, that it is not only founded on dubious and controvertible principles, but that

Heat. that it is, in every point of light, clogged with unfurmountable difficulties.

"I. Many and various are the proofs which evince the improbability of the lungs being the source or laboratory of animal-heat: for, though it be granted, that there subsists a very striking connexion between the state of respiration and the degree of heat in animals, and that they are even in proportion to one another; yet it by no means ensues, that the former is positively the cause of the latter. For, were that really the case, it is obvious, that those animals which are destitute of the organs of respiration would generate no heat. That, however, is not true in fact: for those fishes which are even destitute of gills, appear from various experiments to be warmer than the ordinary temperature of the element in which they live; an irrefragable proof that the function of respiration is not absolutely necessary to the production of heat in animals.

"II. If the heat of living animals be generated solely in the lungs, two things necessarily follow: the first, That it can only be communicated to the other parts of the body through the channel of the arterial system; the second, That the heat must decrease as it recedes from its supposed centre. And a clear and satisfactory evidence of both these points will, no doubt, be deemed requisite to render Dr Black's opinion in any degree probable. So far, however, are we from meeting with those positive and convincing proofs which we had reason to expect, that we are not presented with a single plausible argument in favour of either of the points. On the contrary, it is more conformable to facts, that the venal blood is, if not warmer, at least as warm as the arterial. Dr Stevenson, an ingenious and accurate physiologist, with a view to ascertain this matter, laid bare the jugular vein and carotid artery of a calf, and then tied and cut them off at once, in order to let equal quantities of blood flow, in a given time, into vessels of an equal capacity, in each of which he had placed a well-adjusted thermometer; the result of the experiment was, That the thermometer immersed in the venous blood rose several degrees above that placed in the arterial. But though it is probable that there is not such a difference as that experiment seems to make, yet several reasons incline me to think, that the venous blood, instead of being colder, as Dr Black maintains, is in fact somewhat warmer, than the arterial; and what entirely overturns his opinion is, That no experiment, though many have been made, has ever shewn that the temperature of the blood is higher in the left ventricle of the heart than in the right, which must necessarily be the case, were all the heat of the animal-body generated in the lungs.

"III. Having thus rendered it improbable that the generation of animal-heat should be entirely confined to the lungs, we shall venture a step farther, and endeavour to shew, that the vital fluid, so far from acquiring all its heat in the pulmonary system, communicates no inconsiderable portion of what it had received in the course of the circulation to the air alternately entering into that organ and issuing from it. Various are the arguments which tend to evince this opinion. Were the blood heated in the lungs, we should certainly need less of their function in a warm

than in a cold atmosphere: but we are taught by experience, that when the air is extremely hot, and we wish to be cooled, we breathe full and quick; and that when it is intensely cold, our respiration is slow and languid; which, were the blood heated in the lungs by the action of the air upon it, surely should not be the case. It is therefore more consonant with reason and experience, that the air which we inspire, by carrying off a quantity of evolved phlogiston from the lungs, rather contributes to diminish than increase the heat of breathing animals. Respiration, for this reason, has been very properly compared, by an ingenious physiologist, Dr Duncan of Edinburgh, to the blowing of bellows on a hot body. In both cases a considerable degree of heat is communicated to the air: but in neither can the air be said to generate any heat; for if it did, the heat of breathing animals should increase in proportion to the quantity of air inhaled, and a piece of inert matter heated to a certain degree should become hotter by ventilation.

"IV. The foetus in utero, according to Dr Black's hypothesis, generates no heat. The arguments by which he supports that position, how ingenious soever they may be, seem not sufficiently cogent to produce conviction; and as the question from its nature hardly admits of any direct experiment, our reasoning upon it must necessarily be analogical. Hence arises our embarrassment; for, as the discovering of analogies depends on the quickness and fertility of fancy, and the truth of all analogical reasoning on the acuteness and nicety of judgment, two powers of the soul seldom united in an eminent degree, we cannot wonder that arguments of this kind, which to one man seem unanswerable, should to another appear futile.

"The only plausible objection to the generation of heat in the foetus, is, the supposition that it would in a short time accumulate in such a manner as to become incompatible with life.

"This argument, however, is more specious than solid; for, granting that the circulation which is carried on between the foetus and the mother, transmits very nearly the temperature of her blood, that by no means entirely supercedes the necessity of heat being generated in it. Various reasons lead to this opinion.—It is an axiom, that heat decreases as it recedes from the source from which it sprang. Now, if we admit for a moment Dr Black's opinion, and believe the heat of animals to be generated solely in the lungs, is it not obvious, that before it reaches the uterus, passes through the very minute tubes by which that organ is connected to the placenta, circulates through the umbilical vessels, and pervades the extreme parts of the foetus, it must be too much diminished to support that equilibrium which obtains in every part of the living system. Besides, as the foetus in utero may properly enough be accounted a part of the mother, the same objections that are brought against the generation of heat in it would hold equally good against the production of heat in any part or organ of her body, except the lungs. But such a multitude of accurate thermometrical observations have evinced the partial increase of heat in local inflammations, that no room is left to doubt, that in every individual part of the vital frame heat is generated; and if the foetus be, from any cause whatever, liable to topical inflam-

mation, a thing which no physiologist has ever pretended to deny, what shadow of reason is there for doubting that such affections are accompanied with the same effects before as after birth, and consequently with a partial increase of heat?"

Our author having now, as he supposes, refuted the opinions of others, after shewing that heat though generated cannot accumulate in the fetus, proceeds to lay down his own theory, which depends on the following principles.

1. That the blood does contain phlogiston.

2. That this phlogiston is evolved, extricated, or brought into a state of activity and motion by the action of the blood-vessels to which it is subjected in the course of circulation.

3. That the evolution of phlogiston is a cause which throughout nature produces heat, whether that heat be apparently excited by mixture, fermentation, percussion, friction, inflammation, ignition, or any similar cause.

4. That this heat, which must be produced in consequence of the evolution of the phlogiston from the blood of different animals, is in all probability equal to the highest degree of heat which these animals in any case possess.

The first and second of these propositions will readily be granted: but the third is liable to a very great objection, namely, that from putrefying bodies, phlogiston is evolved in quantity sufficient to reduce to their metallic form the calces of some metals exposed to the vapour, as Dr Duguid hath acknowledged; yet he himself affirms, that no sensible heat is produced by putrefying animal-substances. To this he is obliged to reply, that phlogiston is extricated more slowly from mixtures undergoing the putrid fermentation, than from such as are undergoing the vinous and acetic ones; and that the volatile alkali produced from putrefying substances likewise hinders the action of the phlogiston. But the first part of this answer is not proved, and is what he himself calls only a probable conjecture. Neither doth the second appear to be well founded: for putrefying substances, urine excepted, afford but little volatile alkali; and even putrid urine itself, which affords such a large portion, is not colder than other putrid matters.

It is however needless to insist farther on this theory, since his fundamental principle, namely, That the venous blood is warmer than the arterial, hath been shewn to be false by Mr Adair Crawford, of whose hypothesis we must now give an account.

This gentleman, who, in his general doctrine of heat, seems to agree with Dr Irvin of Glasgow, begins with an explanation of his terms. The words heat and fire, he tells us, are ambiguous. Heat in common language has a double signification. It is used indiscriminately to express a sensation of the mind, and an unknown principle, whether we call it a quality or a substance, which is the exciting cause of that sensation. The latter, he, with Dr Irvin, calls absolute heat; the former, sensible heat. The following are the general facts upon which his experiments are founded.

1. Heat is contained in great quantities in all bodies when at the common temperature of the atmosphere.

2. Heat has a constant tendency to diffuse itself over all bodies, till they are brought to the same de-

gree of sensible heat.

3. If the parts of the same homogeneous body have the same degree of sensible heat, the quantities of absolute heat will be proportionable to the bulk or quantity of matter. Thus the quantity of absolute heat contained in two pounds of water, must be conceived to be double of that which is contained in one pound, when at the same temperature.

4. The mercurial thermometer is an accurate measure of the comparative quantities of absolute heat which are communicated to the same homogeneous bodies or separated from them, as long as such bodies continue in the same form. If therefore the sensible heat of a body, as measured by the mercurial thermometer, were to be diminished the one half, or the one third, or in any given proportion, the absolute heat would be diminished in the same proportion.

5. The comparative quantities of absolute heat which are communicated to different bodies, or separated from them, cannot be determined in a direct manner by the thermometer. Thus, if the temperature of a pound of mercury be raised one degree, and that of a pound of water one degree, as indicated by the thermometer, it does not by any means follow, that equal quantities of absolute heat have been communicated to the water and the mercury. [See HEAT and THERMOMETER.]—If a pint of mercury at 100° be mixed with an equal bulk of water at 50°, the change produced in the heat of the mercury will be to that produced in the water, as three to two: from which it may be inferred, that the absolute heat of a pint of mercury is to that of an equal bulk of water, as two to three; or, in other words, that the comparative quantities of their absolute heats are reciprocally proportionable to the changes which are produced in their sensible heats, when they are mixed together at different temperatures. This rule, however, does not apply to those mixtures which generate sensible heat or cold by chemical action.

From the above position, says Mr Crawford, it follows, that equal weights of heterogeneous substances, as air and water, having the same temperature, may contain unequal quantities of absolute heat. There must, therefore, be certain essential differences in the nature of bodies, in consequence of which some have the power of collecting and retaining the element of fire in greater quantities than others, and these differences he calls throughout his treatise the capacities of bodies for containing heat.

Having premised these general facts, our author gives an account of a number of experiments made, in order to ascertain the quantity of absolute heat contained in different bodies. These experiments were made by mixing the bodies to be examined with water, heated to different degrees; and by the temperature of the mixture, he found the proportion of the capacity of the bodies for containing heat, to water, and, of consequence, to one another. Thus he found the capacity of wheat for containing heat to be to that of water, as 1 to 2.9; and, of consequence, the absolute heats of the two substances to be in the same proportion. The absolute heat of oats to that of water he found as 1 to 2.7; of barley, as 1 to 2.4; of beans, as 1 to 1.6; of flesh, as 1 to 1.3; of milk, as 1 to 1.1; and of a mixture of venous and arterial blood

Heat. blood from a sheep, as 25.4 to 24.4. By other experiments he determined, that the absolute heat of venous blood was to that of water, only as 100 to 112, whereas the absolute heat of arterial blood was to that of water, as 100 to 97.08.

By experiments made with air of different kinds contained in bladders, and immersed in water, he found that the absolute heat of atmospheric air was exceedingly great, being to that of water as 18.6 to 1; that of dephlogisticated air was still greater, being to the heat of common atmospheric air as 4.6 to 1. The heat of phlogisticated and fixed air was much less; that of the latter, particularly, being to the heat of atmospheric air only as 1 to 67.

From other experiments made on metals, Mr Crawford concludes, that the absolute heat of tin, in its metallic state, is to that of water as 1 to 14.7; but the heat of calcined tin is to that of water as 1 to 10.4. In like manner, the heat of iron was to that of water only as 1 to 8; but that of the calx of iron was to the heat of water as 1 to 3.1, &c. And from these experiments he is of opinion, that the more phlogiston that is added to any body, the less is its capacity for containing heat.

From these experiments our author deduces the following theory of animal-heat.—“It has been proved, that the air, which is expired from the lungs of animals, contains less absolute heat than that which is inhaled in inspiration. It has been shown, particularly, that, in the process of respiration, atmospheric air is converted into fixed air; and that the absolute heat of the former is to that of the latter, as 67 to 1.

“Since, therefore, the fixed air which is exhaled by expiration is found to contain only the one sixty-seventh part of the heat which was contained in the atmospheric air previous to inspiration, it follows, that the latter must necessarily deposit a very great proportion of its absolute heat in the lungs. It has moreover been shown, that the absolute heat of florid arterial blood is to that of venous as $11\backslash frac\{1\}\{2\}$ to 10. And hence, as the blood, which is returned by the pulmonary vein to the heart, has the quantity of its absolute heat increased, it is evident that it must have acquired this heat in its passage through the lungs. We may conclude, therefore, that in the process of respiration, a quantity of absolute heat is separated from the air and absorbed by the blood.

“That heat is separated from the air in respiration, is farther confirmed by the experiment with phlogisticated air; from which, compared with Dr Priestley’s discoveries, it is manifest, that the power of any species of air in supporting animal-life, is nearly in proportion to the quantity of absolute heat which it contains, and is consequently proportionable to the quantity which it is capable of depositing in the lungs.

“The truth of this conclusion will perhaps appear in a clearer light from the following calculation, by which we may form some idea of the quantity of heat yielded by atmospheric air when it is converted into fixed air, and also of that which is absorbed during the conversion of venous into arterial blood.

“We have seen, that the same heat, which raises atmospheric air one degree, will raise fixed air nearly 67 degrees; and consequently, that the same

heat, which raises atmospheric air any given number of degrees, will raise fixed air the same number of degrees multiplied by 67. In the Peterburgh experiment of freezing quicksilver, the heat was diminished 200 degrees below the common temperature of the atmosphere. We are therefore certain, that atmospheric air, when at the common temperature of the atmosphere, contains at least 200 degrees of heat. Hence, if a certain quantity of atmospheric air, not in contact with any body that would immediately carry off the heat, should suddenly be converted into fixed air, the heat which was contained in the former would raise the latter 200 degrees multiplied by 67, or 13400 degrees. And the heat of red hot iron being 1050, it follows that the quantity of heat, which is yielded by atmospheric air when it is converted into fixed air, is such, (if it were not dissipated), as would raise the air so changed to more than 12 times the heat of red-hot iron.

“If, therefore, the absolute heat which is disengaged from the air in respiration, were not absorbed by the blood, a very great degree of sensible heat would be produced in the lungs.

“Again, it has been proved, that the same heat which raises venous blood 115 degrees, will raise arterial only 100 degrees; and consequently, that the same heat, which raises venous blood any given number of degrees, will raise arterial a less number, in the proportion of 100 to 115, or 20 to 23. But we know that venous blood contains at least 230 degrees of heat. Hence, if a certain quantity of venous blood, not in contact with any body that would immediately supply it with heat, should suddenly be converted into arterial, the heat which was contained in the former would raise the latter only $\backslash frac\{2\}\{3\}$ of 230 degrees, or 200 degrees; and consequently the sensible heat would suffer a diminution, equal to the difference between 230 and 200, or 30 degrees. But the common temperature of blood is 96; when, therefore, venous blood is converted into arterial in the lungs, if it were not supplied by the air with a quantity of heat proportionable to the change which it undergoes, its sensible heat would be diminished 30 degrees, or it would fall from 96 to 66.

“That a quantity of heat is detached from the air, and communicated to the blood, in respiration, is moreover supported by the experiments with metals and their calces: from which it appears, that when bodies are joined to phlogiston, they lose a portion of their absolute heat; and that, when the phlogiston is again disengaged, they reabsorb an equal portion of heat from the surrounding bodies.

“Now it has been demonstrated by Dr Priestley, that in respiration, phlogiston is separated from the blood and combined with the air. During this process, therefore, a quantity of absolute heat must necessarily be disengaged from the air, by the action of the phlogiston; the blood, at the same moment, being left at liberty to unite with that portion of heat which the air had deposited.

“And hence animal-heat seems to depend upon a process similar to a chemical elective attraction. The air is received into the lungs, containing a great quantity of absolute heat. The blood is returned from the

the extremities, highly impregnated with phlogiston. The attraction of the air to the phlogiston, is greater than that of the blood. This principle will, therefore, leave the blood to combine with the air. By the addition of the phlogiston, the air is obliged to deposit a part of its absolute heat; and as the capacity of the blood is at the same moment increased by the separation of the phlogiston, it will instantly unite with that portion of heat which had been detached from the air.

"We learn from Dr Priestley's experiments with respect to respiration, that arterial blood has a strong attraction to phlogiston: it will consequently, during the circulation, imbibe this principle from those parts which retain it with least force, or from the putrescent parts of the system: and hence the venous blood, when it returns to the lungs, is found to be highly impregnated with phlogiston. By this impregnation, its capacity for containing heat is diminished. In proportion, therefore, as the blood, which had been dephlogisticated by the process of respiration, becomes again combined with phlogiston in the course of the circulation, it will gradually give out that heat which it had received in the lungs, and diffuse it over the whole system.

"Thus it appears, that, in respiration, the blood is continually discharging phlogiston and absorbing heat; and that, in the course of the circulation, it is continually imbibing phlogiston and emitting heat.

"It may be proper to add, that as the blood, by its impregnation with phlogiston, has its capacity for containing heat diminished; so on the contrary, those parts of the system from which it receives this principle, will have their capacity for containing heat increased, and will consequently absorb heat.

"Now if the changes in the capacities, and the quantities of matter changed in a given time, were such, that the whole of the absolute heat separated from the blood were absorbed, it is manifest that no part of the heat which is received in the lungs would become sensible in the course of the circulation.

"That this, however, is not the case, will, I think, be evident from the following considerations:

"We know that sensible heat is produced by the circulation of the blood; and we have proved by experiment, that a quantity of absolute heat is communicated to that fluid in the lungs, and is again disengaged from it in its progress through the system. If, therefore, the whole of the absolute heat, which is separated from the blood, were absorbed by those parts of the system from which it receives the phlogiston, it would be necessary to have recourse to some other cause, to account for the sensible heat which is produced in the circulation. But, by the rules of philosophising, we are to admit no more causes of natural things than such as are both true and sufficient to explain the appearances; for nature delights in simplicity, and affects not the pomp of superfluous causes.

"We may, therefore, safely conclude, that the absolute heat which is separated from the air in respiration, and absorbed by the blood, is the true cause of animal-heat.

"It must nevertheless be granted, that those parts of the system which communicate phlogiston to the

blood, will have their capacity for containing heat increased; and therefore, that a part of the absolute heat which is separated from the blood will be absorbed.

"But from the quantity of heat, which becomes sensible in the course of the circulation, it is manifest that the portion of heat which is thus absorbed is very inconsiderable.

"It appears, therefore, that the blood, in its progress through the system, gives out the heat which it had received from the air in the lungs: a small portion of this heat is absorbed by those particles which impart the phlogiston to the blood; the rest becomes redundant, or is converted into moving and sensible heat."

Mr Crawford's theory, which doth not essentially differ from Dr Black's, seems to be the best that hath yet appeared. There is, however, one difficulty which seems common to them all, and which, even on Mr Crawford's principles, seems not to admit of solution. If animal-heat entirely depends on something peculiar to a living body, why doth it sometimes continue after life hath ceased? If heat depends on the evolution of phlogiston by the action of the blood-vessels, according to Dr Dugud, why should it remain when these vessels cease to act, as, according to Dr Dugud himself, it sometimes doth? If, according to Mr Crawford, it is every moment attracted from the air, why is it not always in proportion to the respiration? Or, if fixed air contains such a small proportion of absolute heat as, by Mr Crawford's experiments, it seems to do, why doth it impart such a strong and lasting degree of heat to the bodies of those who are killed by it? See the article BLOOD, n^o 31. The conjecture mentioned under that article, n^o 32, is therefore still probable, namely, that animal-heat is occasioned by the elastic principle of fixed air; tho' in what manner it is occasioned, seems to be problematical.

Internal Heat of the Earth. That there is a very considerable degree of heat always felt in digging to great depths in the earth, is agreed upon by all naturalists: but the quantity of this heat hath seldom been measured in any part; much less is it known, whether, in digging to an equal depth in different parts of the earth, the heat is found always the same. In digging mines, wells, &c. they find that at a little depth below the surface it feels cold. A little lower it is colder still, as being beyond any immediate influence of the sun's rays; inasmuch, that water will freeze almost at any season of the year: but when we go to the depth of 40 or 50 feet, it begins to grow warm, so that no ice can bear it; and then the deeper we go, still the greater the heat, until at last respiration grows difficult, and the candles go out.

This heat of the earth hath been variously explained. Some have had recourse to an immense body of fire lodged in the centre of the earth, which they consider as a central sun, and the great principle of the generation, vegetation, nutrition, &c. of fossil and vegetable bodies. But Mr Boyle, who had been at the bottom of some mines himself, suspects that this degree of heat, at least in some of them, may arise from the peculiar nature of the minerals generated therein. To confirm this, he instances a mineral of a vitriolic kind, dug up in large quantities in many parts of England, which by the bare affusion of common water will grow

Heat. so hot, that it will almost take fire.—These hypotheses are liable to the following objections. 1. If there is within the earth a body of actual fire, it seems difficult to shew why that fire should not consume and moulder away the outer shell of earth, till either the earth was totally destroyed, or the fire extinguished. 2. If the internal heat of the earth is owing to the action of water upon mineral substances, that action thro' time must have ceased, and the heat have totally vanished; but we have no reason to think, that the heat of the earth is any thing less just now than it was a thousand years ago. The phenomenon is easily explained by the propositions laid down under the article HEAT. If heat is nothing else than a certain mode of action in the ethereal fluid, or the matter of light, by which it flows out from a body in all directions as radii drawn from the centre to the circumference of a circle; it will then follow, that if an opaque body absorbs any considerable quantity of light, it must necessarily grow hot. The reason of this is plain. The body can hold no more than a certain quantity of ethereal matter; if more is continually forcing itself in, that which has already entered must go out. But it cannot easily get out, because it is hindered by the particles of the body among which it is detained. It makes an effort therefore in all directions to separate these particles from each other; and hence the body expands, and the effort of the fluid to escape is felt when we put our hands on the body, which we then say is hot. Now as the earth is perpetually absorbing the ethereal matter, which comes from the sun in an immense stream, and which we call his light, it is plain, that every pore of it must have been filled with this matter long ago. The quantity that is lodged in the earth, therefore, must be continually endeavouring to separate its particles from each other, and consequently must make it hot. The atmosphere, which is perpetually receiving that portion of the ethereal matter which issues from the earth, counteracts the force of the internal heat, and cools the external surface of the earth, and for a considerable way down; and hence the earth for 20 or 30 feet down, shews none of that heat which is felt at greater depths. See HEAT.

Heat, in medicine. Great heats are not so much the immediate, as the remote, cause of a general sickness, by relaxing the fibres, and disposing the juices to putrefaction; especially among soldiers and persons exposed the whole day to the sun: for the greatest heats are seldom found to produce epidemic diseases, till the perspiration is stopped by wet clothes, fogs, dews, damps, &c.; and then some bilious or putrid distemper is the certain consequence, as fluxes and ardent intermitting fevers. Nevertheless, it must be allowed, that heats have sometimes been so great as to prove the more immediate cause of particular disorders; as when centinels have been placed without cover, or frequent reliefs in scorching heats; or when troops march or are exercised in the heat of the day; or when people imprudently lie down and sleep in the sun. All these circumstances are apt to bring on distempers, varying according to the season of the year. In the beginning of summer, these errors produce inflammatory fevers; and in autumn, a remitting fever or dysentery. To prevent, therefore, the effects of immoderate heats, commanders have found it expedient so to order the

marches, that the men come to their ground before the heat of the day; and to give strict orders, that none of them sleep out of their tents, which, in fixed encampments, may be covered with boughs to shade them from the sun. It is likewise a rule of great importance to have the soldiers exercised before the cool of the morning is over; for by that means not only the sultry heats are avoided, but the blood being cooled, and the fibres braced, the body will be better prepared to bear the heat of the day. Lastly, in very hot weather, it has often been found proper to shorten the centinels duty, when obliged to stand in the sun.