550 



CHAPTER II. 

 THE LAWS OF CHANGES OCCASIONED BY HEAT. 



Sect. 1. Expansion by Heat. The Law of Dalton 

 and Gay-Lussac for Gases. 



A LMOST all bodies expand by heat; solids, as 

 _LJL 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 sub- 

 stance. Mercury appears to be the liquid which is 

 most uniform in its thermometrical march ; and has 

 been taken as the most common material of our 

 thermometers ; but the expansion of mercury is not 



