“Law 7. The cooling power of a gas varies with its temperature in such a manner that if the gas can dilate so as to preserve the same uniform tension, the cooling power will be as much diminished by the rarefaction of the gas, as it is increased by its augmentation of temperature; so that definitively it depends only on its tension.”
Another ingenious Essay was published by Messrs. Dulong and Petit, in the Annal. de chimie et de physique, vol. 10, namely, “Researches on some important points of the theory of heat.”—One object is to ascertain the specific heats of bodies with superior precision. A table of the specific heats of certain metals, found by their method, is given, together with the weights of the atoms of those metals, and the products of the specific heats and weights of the atoms, as under:
| Specific heats, that of water being 1 | Weights of the atoms, that of oxygen being 1 | Product of the weight of each atom by the corresponding capacity. | |
|---|---|---|---|
| Bismuth | 0.0288 | 13.300 | 0.3830 |
| Lead | 0.0293 | 12.950 | 0.3794 |
| Gold | 0.0298 | 12.430 | 0.3704 |
| Platinum | 0.0314 | 11.160 | 0.3740 |
| Tin | 0.0514 | 7.350 | 0.3779 |
| Silver | 0.0557 | 6.750 | 0.3759 |
| Zinc | 0.0927 | 4.030 | 0.3736 |
| Tellurium | 0.0912 | 4.030 | 0.3675 |
| Copper | 0.0949 | 3.957 | 0.3755 |
| Nickel | 0.1035 | 3.690 | 0.3819 |
| Iron | 0.1100 | 3.392 | 0.3731 |
| Cobalt | 0.1498 | 2.460 | 0.3685 |
| Sulphur | 0.1880 | 2.011 | 0.3780 |
The inference intended from this Table is pretty obvious, namely, that the atoms or ultimate particles of the above bodies contain or attach to themselves the same quantity of heat, or have the same capacity. This principle the authors think will apply to the simple atoms of all bodies, whether solid, liquid, or elastic; but they hold it does not apply to compound atoms. It differs therefore essentially from a suggestion of mine, made eighteen years ago, (see Vol. I. page 70,) that the quantity of heat belonging to the ultimate particles of all elastic fluids, must be the same under the same pressure and temperature. They seem to apprehend, from experience, that a very simple ratio exists between the capacities of compound atoms and that of the elementary atoms. They draw another inference from their researches, that the heat developed at the instant of the combination of bodies, has no relation to the capacity of the elements; this loss of heat, they argue, is often not followed by any diminution in the capacity of the compounds. They seem to think that electricity developes heat in the act of combination; but they do not deny that a change of capacity may sometimes ensue, and heat be developed from this cause.
Remarks on the above Essays.
Results nearly agreeing with those of De la Roche and Berard, on the capacity of certain elastic fluids for heat, were about the same time obtained by M. M. Clement and Desormes. (See Journal de Physique, Vol. 89—1819.) Such results, impugning some of the most plausible doctrines of heat, could not be admitted but upon very good authority. I remained doubtful, in some degree, till satisfied by my own experience. I procured a calorimeter of the construction of De la Roche’s, and to simplify the experiment, instead of forcing a given volume of hot air through the calorimeter to impart heat to the water, I drew, by means of an air-pump, a certain volume of atmospheric (or other air) of the common temperature, through the calorimeter filled with hot water, in order to find how much this process would accelerate the cooling. From several experiments of this kind, I am convinced that the capacity of common air for heat is very nearly such as the above ingenious French chemists have determined. That is, it is about ¹/₇ part only of what Dr. Crawford deduced from his experiments, and nearly the same part of what I inferred from my theoretic view of the specific heats of elastic fluids. (See Vol. I. pages 62 and 74.)
Indeed M. M. De la Roche and Berard appear to have been puzzled with the admission of their own results. The combined heats of oxygen and hydrogen gases give only .6335 for the specific heat of water; whereas by experiment the heat of water is found to be 1, notwithstanding an immensity of heat is evolved during the combination of these gases.[26]
“It is necessary therefore,” they observe, “to abandon the hypothesis which ascribes the evolution of heat in cases of combination to a diminution of specific heat in the bodies combined, and admit with Black, Lavoisier, and Laplace, and many other philosophers, the existence of caloric in a state of combination in bodies.” I am not aware of any writer that denies the existence of caloric in a state of combination of bodies. Dr. Crawford, who would be thought the most likely to err in this respect, maintains, “that elementary fire is retained in bodies partly by its attraction to those bodies and partly by the action of the surrounding heat,” and that “its union with bodies will resemble that particular species of chemical union wherein the elements are combined by the joint forces of pressure and of attraction.” (On animal heat, 2d edition, page 436.) He is perhaps somewhat unfortunate in his instance in the combination of carbonic acid and water; muriatic acid or ammonia and water would have been more in point.
The truth is, these important experiments shew that in elastic fluids the increments of temperature are not proportional to the whole heat, compared with the like increments of temperature and whole heat in those bodies when in the liquid and solid states.