CHANGES OCCASIONED BY HEAT. 157 



CHAPTER II. 



THE LAWS OF CHANGES OCCASIONED BY HEAT. 



Sect. 1. Expansion ly Heat. The Laic of DaUon and Gay-Lussuc 



for Gases. 



\ LMOST all bodies expand by heat ; solids, as metals, in a small 

 H- 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 Avas 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 dif- 

 ferent 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 mer- 

 cury 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 success- 

 ful. 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 expvml 

 b// the same fraction of their own lulk ; which fraction is tkree-eir/hths 



