Thus one step was gained, and it was universally admitted that heat, in some cases, entered bodies, and in others was thrown out of them; but now the question arose, What is heat; and by what laws is it regulated, or from what source is it derived? Here Dr. Black himself was at a loss; for, as he supposed cold to be a mere non-entity, and only to consist in a comparatively smaller degree of heat, some phenomena occurred which would not easily admit of solution upon such an hypothesis. With these Dr. Black did not meddle much, but others were bolder. Dr. Irving, Professor of Chemistry at Glasgow, undertook to explain the whole mystery of latent heat upon the single principle of attraction. One of the most puzzling phenomena in the way of Dr. Black’s theory had been, that in some cases heat and cold seemed to repel each other, and a very remarkable instance of this was, that, in the morning, a little before sunrise, when the rays of light pass through the atmosphere, a little above the surface of the earth, the air then becomes manifestly colder than even at midnight. Dr. Irving’s explanation of this was, that the sun’s rays attracted heat from the atmosphere, and thus rendered it colder. Such at least was the explanation given in an inaugural dissertation by Dr. Cleghorn, one of Dr. Irving’s scholars; for the Doctor himself delivered his opinions only to them. In other cases he supposed that different substances had different capacities for receiving heat; and, of consequence, should the form, or rather the internal constitution, of the body be changed, the capacity of it for receiving heat must also be changed; and as an attraction subsists, or is supposed to subsist, between heat and all other substances, it is plain that while this attraction subsists, if the capacity of any substance for receiving heat be augmented, it will imbibe much more than it would have done had its former constitution remained. Thus water in its liquid state contains a certain quantity of heat; we may therefore say that water has a capacity for receiving heat equal to one to ten, or what we please. Vapour has a capacity for containing heat ten times greater than water. Water therefore, when converted into vapour, will imbibe ten times the quantity of heat that the water contains; and, again, on being re-converted into water, the capacity becoming what it was before, the superfluous quantity must be thrown out, as in Dr. Black’s experiments. In like manner, when a metal is melted by the fire, the capacity of it for receiving heat is changed: of consequence a great quantity is imbibed, and again expelled by the change of capacity which takes place on its becoming solid; and thus, from the change of capacity, in different substances, every phenomenon was solved.
This doctrine of capacities did not give general satisfaction. Dr. Black himself said of it, that it was neither probable nor ingenious;[75] notwithstanding which, it continued to be received, and even very generally adopted. Dr. Crawford, so well known for his writings on this subject, has adopted the idea, and Dr. Girtanner, in the passage above quoted from him, appears to be of the same opinion. The doctrine, however, had several opponents, among whom were the Monthly Reviewers. In their account of Nicholson’s First Principles of Chemistry, they express themselves in the following manner: “We only wish, that, in the doctrine of heat, he had avoided, which he might easily have done, Dr. Crawford’s idea of bodies having different capacities for heat. In the melting of ice, for instance, a quantity of heat is absorbed, without any increase of the temperature, that is, without making the water sensibly warmer than the ice before its liquefaction; which is said to be owing to the water having a greater capacity for heat, or being able to hold more of it, than the ice; and, in like manner, when converted into vapour, its capacity is further increased, or it can hold more still. This appears to us a very unchemical, and a very inadequate idea of the matter: for, admitting water to have a greater capacity than ice, how is the change from one state to the other to be effected? Can the properties which a body is found to possess, after a change has taken place, be assigned as a cause of the change itself? Or will it be said, that the heat first enlarges the capacity, and then hides itself in that capacity so enlarged? We should think it much better to say, consonantly with the phenomena of other combinations in chemistry, that a certain quantity of heat, uniting with the ice, first liquefies it, as a certain quantity of acid only neutralizes an alkali; that if any surplus quantity must be introduced, that surplus, remaining free and uncombined, must act and be sensible as heat in the one case, and acid other; and that different bodies require different quantities of heat or acid to be combined with them, for producing the changes in question.”[76]
Thus the Reviewers, as well as others, reasoned a priori, and several facts were adduced to prove that no such changes in capacity could take place. But however strong the arguments adduced, or however plain the experiments might be, little or no notice was taken of them, and the enlargement or diminution of capacities has been repeated, seemingly by rote, from one author to another, without the least inquiry or investigation. Dr. Girtanner indeed says that “the oxygen united with the arterial blood in the lungs” is the cause of the great capacity of the arterial blood for heat. But this is assigning a very doubtful cause for a very doubtful effect. He ought to have proved in the first place that arterial blood really has this capacity; for its being hotter than the blood of the veins, only shews that it parts with more heat to surrounding bodies than venous blood does; which is a proof that it contains less heat, if there be any difference, than that of the veins. But the truth is, that the capacity for containing heat depends neither on the oxygenation nor hydrogenation of a fluid, but upon its density. The more fluid and the more easily expansible into vapour that any substance is, the greater quantity of heat it is capable of containing, and vice versa. This has been fully ascertained by Mr. William Jones, an English clergyman, whose observations on the generally received system of philosophy contain many particulars worthy of attention. From his experiments it appears that a piece of red-hot iron, thrown into water, imparts much less sensible heat to it, and is itself much more effectually quenched, than by throwing it into an equal quantity of quick-silver of the same temperature with the water. As the quick-silver therefore becomes much hotter to the touch than water does upon throwing a piece of red-hot iron into it, and as the iron itself is much more imperfectly quenched by the metal than by the water, it follows that the latter is capable of containing much more heat than the former. But such experiments are not applicable to the blood. Though that of the arteries may be somewhat hotter than the venous blood, yet the reason is obvious. The heat is communicated directly to the arterial blood in the lungs; but during the circulation a part of it evaporates, and the farther distant any part is from the lungs, the more cool will the vital fluid be, without regard to any alteration of capacity, which indeed never can be shown to exist.
But the most decisive experiments against any supposed alteration in the capacities of bodies for containing heat are those lately tried by Count Rumford, and related in the Philosophical Transactions for 1798. His attention to this subject was engaged by observing the great degree of heat acquired by a brass gun during the time of boring it,[77] and still more by the intense heat (much greater than that of boiling water) of the metallic chips separated from it by the borer. From a consideration of these things he was naturally led to the following inquiries. “Whence comes the heat actually produced in this mechanical operation? Is it furnished by the metallic chips which are separated by the borer from the solid mass of metal? If this were the case, then, according to the modern doctrine of caloric, the capacity for heat of the parts of the metal so reduced to chips, ought not only to be changed, but the change undergone by them be sufficiently great to account for all the heat produced. But no such change had taken place; for I found, that by taking equal quantities by weight of these chips, and of thin slips of the same block of metal, separated by means of a fine saw, and putting them at the same temperature, that of boiling water, and putting them into equal quantities of cold water (that is to say, at 59 1/2 of Fahrenheit) the portion of water into which the chips were put, was not, to all appearance, heated either less or more than the other portion in which the chips were put.”
From this experiment, several times repeated with the same result, Count Rumford inferred, that the heat could not possibly have been furnished at the expense of the latent heat of the metallic chips. He then proceeded to ascertain “how much heat was actually generated by friction, when a blunt steel borer being so forcibly shoved (by means of a strong screw) against the bottom of the bore of the cylinder, [of the machine in use] that the pressure against it was equal to the weight of about ten thousand lb. avoirdupois, the cylinder being turned round on its axis (by the power of horses) at the rate of about thirty-two times in a minute.” In this experiment the metallic dust or scaly matter detached from the cylinder by the borer weighed only 837 grains troy; but, says the author, “Is it possible that the very considerable quantity of heat produced in this experiment (a quantity which actually raised the temperature of above 113 lb. of gun-metal at least 70 degrees of Fahrenheit’s thermometer, and which of course would have been capable of melting 6 1/2 lbs. of ice, or making near five pounds of ice-cold water to boil) could have been furnished by so inconsiderable a quantity of metallic dust, and this merely in consequence of a change of its capacity for heat? As the weight of this metallic dust (837 grains troy) amounted to no more than one 948th part of that of the cylinder, it must have lost no less than 948 degrees of heat to have been able to raise the temperature of the cylinder one degree; and consequently it must have given off more than sixty-six thousand, three hundred and sixty degrees of heat to have produced the effects which were actually found to have been produced in this experiment.”
It was next considered whether the air did not contribute to the generation of this heat; and our author determined that this could not be the case; because the quantity of heat generated was not sensibly diminished when the free access of air was prevented. From another experiment it appeared that the generation of the heat was neither prevented nor retarded by keeping the apparatus immersed in water. Here the friction generated so much heat, that in one hour the temperature of the water surrounding the cylinder was raised from 60 to 107 degrees of Fahrenheit. In half an hour more it was raised to 142; at the end of two hours to 178; at two hours 20 minutes to 200; and in two hours and a half it boiled.[78] On the whole, Count Rumford concludes, that “the quantity of heat, produced equably by the friction of the blunt borer against the bottom of the hollow metallic cylinder, was greater than that produced equably in the combustion of nine wax candles, each three quarters of an inch diameter, all burning at the same time with a clear, bright flame.” From all these experiments, however, our author does not draw any certain conclusion. “What is heat? (says he.) Is there any such thing as an igneous fluid? Is there any thing that can with propriety be called caloric? The heat produced, in the author’s experiments, by the friction of two metallic surfaces, was not furnished by small particles of metal, detached from the larger solid on their being rubbed together. It was not supplied by the air, because the machinery in three experiments was kept under water, and the access of atmospherical air completely prevented. It was not furnished by the water which surrounded the machinery, because this water was continually receiving heat from the machinery and could not at the same time be giving to and receiving heat from the same body; and because there was no chemical decomposition of any part of this water.” At last he observes, that the source of this heat, whatever it is, must evidently be inexhaustible, adding, that “any thing, which any insulated body, or system of bodies, can continue to furnish without limitation, cannot possibly be a material substance; and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of any thing capable of being excited and communicated, in the manner the heat was excited and communicated in these experiments, except it be MOTION.”
On this last paragraph, however, it is obvious to remark, that the whole force of the argument rests upon an insinuation, that the cylinder and borer were insulated, or cut off from all communication with any other material substance. Had this been the case, then no doubt it would follow that an endless supply of any thing material could not be furnished by them; but if, as Dr. Boerhaave and many other learned and intelligent persons have supposed, fire be an element universally present, and which becomes sensible to the touch only in consequence of a particular mode of action, it will follow, that no substance in nature can be insulated with respect to it; but, in whatever place, and for whatever length of time, any substance shall be affected in such a manner as to agitate this fluid, there we shall perceive a production of heat without limitation, even though heat itself be no more than the action of a fluid essentially material, though invisible to us.
Considerations of this kind occurred long ago to the writer of this treatise, when by the nature of his employment it was necessary for him to speculate upon these subjects. It could not then but appear to him that the theory of Dr. Black was far superior to any that had been published. The opinion of those who supposed fire to consist in the vibratory motion of the particles of solid bodies, seemed altogether untenable. It is impossible to explain the phenomena of heat upon ordinary mechanical principles, because, with respect to all terrestrial substances, heat constantly appears as an agent, while they are merely passive; and no man can explain the nature of a cause from its effect. Thus one of the most obvious effects of heat is expansion, or enlargement of bulk, in such bodies as are heated. But if from this fact we infer that the parts of elementary fire are repulsive of one another, our reasoning is certainly erroneous. In like manner, when we are not sensible of heat, we are not authorised to conclude that it is not present; for Dr. Black has demonstrated that it may be present in very great quantity, though indiscoverable either by our senses or by a thermometer.
But, with regard to the theory published by Dr. Black himself, it is evident that, though one part of it rests on the solid basis of experiment, the other is founded entirely upon hypothesis, and that too an hypothesis which cannot admit of being proved by any experiment, viz. that cold is a mere negative, and hath no real existence in nature. Among many phenomena which militate against this opinion, the following experiment of M. Geoffrey seems to be the most remarkable. He took a small bason filled with water, and set it on a support in the middle of a large tub of water, in such a manner that the temperature of the water in the tub might communicate itself to that in the bason. This being ascertained by a thermometer placed in the bason, he threw a quantity of burning coals into the tub. The effect of this, on the supposition that cold is a mere privation of heat, ought to have been, that the heat of the coals, communicated to the water in the large tub, would in a short time pervade the small bason, and affect the thermometer there. The latter would therefore rise; but instead of this it fell several degrees before it began to rise; for which it doth not appear that any other reason can be assigned than that the cold is partly repelled by the heat of the coals, and therefore, entering into the small bason of water, it causes the thermometer to sink previous to its rising. To the same purpose we may urge the phenomenon already taken notice of, viz. that the sun’s rays, when passing at some distance above the surface of the earth, cool the lower part of the atmosphere. The natural solution is, that the heat of the sun partly repels the cold downwards; and as for the doctrine of attracting heat from the atmosphere, Count Rumford has shown that this does not happen in a case where we might with much more probability expect it; not to mention the violence done to the common perceptions of mankind by supposing the sun’s rays, which are most evidently the source of heat, to have any occasion to attract heat from the atmosphere or any thing else.
Lastly, with regard to the capacities of bodies for containing heat, the doctrine appears to involve a radical error, of such enormous magnitude, that it is impossible to make any thing of it. This is no less than confounding the heat which flows out from bodies with that which they contain as an essential part of their composition, and which they cannot emit without being changed into some other form. Thus the capacity of aqueous vapour for containing heat, according to Dr. Black, is 1000 degrees; yet without decomposing the vapour it would have been impossible to have known this; for vapour is often extremely cold to the touch, and a thermometer immersed in it will sink greatly. In short, all that we can know about the capacity of bodies for retaining heat is, that they either continue to absorb it, or we may continue to force it into them, till they be reduced to vapour. It is doubtful whether they can receive more; for from the experiment with Papin’s digester, formerly mentioned, it appears that the additional quantity of heat, which the water was made to receive, very quickly left it as soon as the steam had room to expand.