552 HISTORY OF THERMOTICS. 



does not depend upon physical properties, and I 

 collect that all gases expand equally by heat" He 

 then extends this to vapours, as ether. This must 

 be one of the most important foundation-stones of 

 any sound theory of heat (QA). 



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. It 

 now appears that this result is independent of any 

 peculiar properties in the air employed; and thus 

 this measure has an additional character of gene- 

 rality and simplicity which make it still more pro- 

 bable 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 capac'itu 

 for heat, according to the quantity of heat which it 

 required to raise it through a given scale of heat 3 . 



3 See Crawfurd, On Heat ; for the History of Specific Heat. 



