[32] Manch. Mem. vol. v. 1802; and Ann. Chim. xliii. p. 137.
[33] Ib. p. 272.
[2nd Ed.] Yet MM. Magnus and Regnault conceive that they have overthrown this law of Dalton and Gay-Lussac, and shown that the different gases do not expand alike for the same increment of heat. Magnus found the ratio to be for atmospheric air, 1∙366; for hydrogen, 1∙365; for carbonic acid, 1∙369; for sulphurous-acid gas, 1∙385. But these differences are not greater than the differences obtained for the same substances by different observers; and as this law is referred to in Laplace’s hypothesis, [hereafter] to be discussed, I do not treat the law as disproved.
Yet that the rate of expansion of gas in certain circumstances is different for different substances, must be deemed very probable, after Dr. Faraday’s recent investigations On the Liquefaction and Solidification of Bodies generally existing as Gases,[34] by which it appears that the elasticity of vapors in contact with their fluids increases at different rates in different substances. “That the force,” he says, “of vapor increases in a geometrical ratio for equal increments of heat is true for all bodies, but the ratio is not the same for all. . . . For an increase of pressure from two to six atmospheres, the following number of degrees require to be added to the bodies named:—water 69°, sulphureous acid 63°, cyanogen 64°∙5, ammonia 60°, arseniuretted hydrogen 54°, sulphuretted hydrogen 56°∙5, muriatic acid 43°, carbonic acid 32°∙5, nitrous oxide 30°.”]
[34] Phil. Trans. 1845, Pt. 1.
We have [already] seen that the opinion that the air-thermometer is a true measure of heat, is strongly countenanced by the symmetry which, by using it, we introduce into the laws of radiation. If we [159] accept the law of Dalton and Gay-Lussac, it follows that this result is independent of any peculiar properties in the air employed; and thus this measure has an additional character of generality and simplicity which make it still more probable that it is the true standard. This opinion is further supported by the attempts to include such facts in a theory; but before we can treat of such theories, we must speak of some other doctrines which have been introduced.
Sect. 2.—Specific Heat.—Change of Consistence.
In the attempts to obtain measures of heat, it was found that bodies had different capacities for heat; for the same quantity of heat, however measured, would raise, in different degrees, the temperature of different substances. The notion of different capacities for heat was thus introduced, and each body was thus assumed to have a specific capacity for heat, according to the quantity of heat which it required to raise it through a given scale of heat.[35] The term “capacity for heat” was introduced by Dr. Irvine, a pupil of Dr. Black. For this term, Wilcke, the Swedish physicist, substituted “specific heat;” in analogy with “specific gravity.”
[35] See Crawford, On Heat, for the History of Specific Heat.
It was found, also, that the capacity of the same substance was different in the same substance at different temperatures. It appears from experiments of MM. Dulong and Petit, that, in general, the capacity of liquids and solids increases as we ascend in the scale of temperature.