[30] See Prof. Forbes’s Third Series of Researches on Heat, Edinb. R.S. Trans. vol. xiv.
M. Melloni has assumed this analogy as so completely established, that he has proposed for this part of thermotics the name Thermochroology (Qu. Chromothermotics?); and along with this term, many others derived from the Greek, and founded on the same analogy. If it should appear, in the work which he proposes to publish on this subject, that the doctrines which he has to state cannot easily be made intelligible without the use of the terms he suggests, his nomenclature will obtain currency; but so large a mass of etymological innovations is in general to be avoided in scientific works.
M. Melloni’s discovery of the extraordinary power of rock-salt to transmit heat, and Professor Forbes’s discovery of the extraordinary power of mica to polarize and depolarize heat, have supplied thermotical inquirers with two new and most valuable instruments.[31]]
[31] For an account of many thermotical researches, which I have been obliged to pass unnoticed here, see two Reports by Prof. Powell on the present state of our knowledge respecting Radiant Heat, in the Reports of the British Association for 1832 and 1840.
Moreover, besides the laws of conduction and radiation, many other laws of the phenomena of heat have been discovered by philosophers; and these must be taken into account in judging any theory of heat. To these other laws we must now turn our attention. [157]
CHAPTER II.
The Laws of Changes occasioned by Heat.
Sect. 1.—Expansion by Heat.—The Law of Dalton and Gay-Lussac for Gases.
ALMOST all bodies expand by heat; solids, as metals, in a small degree; fluids, as water, oil, alcohol, mercury, in a greater degree. This was one of the facts first examined by those who studied the nature of heat, because this property was used for the measure of heat. In the Philosophy of the Inductive Sciences, Book iv., Chap. iv., I have stated that secondary qualities, such as Heat, must be measured by their effects: and in Sect. 4 of that Chapter I have given an account of the successive attempts which have been made to obtain measures of heat. I have there also spoken of the results which were obtained by comparing the rate at which the expansion of different substances went on, under the same degrees of heat; or as it was called, the different thermometrical march of each substance. Mercury appears to be the liquid which is most uniform in its thermometrical march; and it has been taken as the most common material of our thermometers; but the expansion of mercury is not proportional to the heat. De Luc was led, by his experiments, to conclude “that the dilatations of mercury follow an accelerated march for equal augmentations of heat.” Dalton conjectured that water and mercury both expand as the square of the real temperature from the point of greatest contraction: the real temperature being measured so as to lead to such a result. But none of the rules thus laid down for the expansion of solids and fluids appear to have led, as yet, to any certain general laws.
With regard to gases, thermotical inquirers have been more successful. Gases expand by heat; and their expansion is governed by a law which applies alike to all degrees of heat, and to all gaseous fluids. The law is this: that for equal increments of temperature they expand by the same fraction of their own bulk; which fraction is three-eights [158] in proceeding from freezing to boiling water. This law was discovered by Dalton and M. Gay-Lussac independently of each other;[32] and is usually called by both their names, the law of Dalton and Gay-Lussac. The latter says,[33] “The experiments which I have described, and which have been made with great care, prove incontestably that oxygen, hydrogen, azotic acid, nitrous acid, ammoniacal acid, muriatic acid, sulphurous acid, carbonic acid, gases, expand equally by equal increments of heat.” “Therefore,” he adds with a proper inductive generalization, “the result does not depend upon physical properties, and I collect that all gases expand equally by heat.” He then extends this to vapors, as ether. This must be one of the most important foundation-stones of any sound theory of heat.