84 



HEAT. 



specific heat x atomic weight is very nearly constant for different 

 gases and independent of the temperature, when the specific heat is 

 taken in all cases at constant pressure, or in all cases at constant 

 volume. Again, for a very large number of solid elements the pro- 

 duct is not far from constant, though the constant is not the same as 

 for gases. 



These relations imply that if we take quantities of different ele- 

 mentary substances proportional to their atomic weights, and therefore 

 presumably containing the same number of atoms, the heat capacities of 

 these quantities will be nearly equal. Or the heat capacity per atom is 

 the same for different elements, the gas atom, however, having a 

 different capacity from the solid atom. The product atomic weight x 

 specific heat is termed the " atomic heat." 



In many classes of compounds, too, the product specific heat x 

 molecular weight, the " molecular heat," is not far from constant, each 

 class having as a rule its own constant, though in some cases each 

 constituent atom may be regarded as having its atomic heat as a gas if 

 the compound is gaseous, as a solid if it is solid. 



Without entering into minute detail, we shall give examples of the 

 evidence on which these statements are founded. 



Taking first the case of gaseous bodies. Regnault found by the 

 method already described that at constant pressure : 



1. The specific heat is nearly independent of the temperature. 



2. It is nearly independent of the pressure so long as this is constant 



during an experiment. 



3. The capacities for heat of equal volumes of different gases at 



equal pressures are nearly equal whence on the molecular 

 hypothesis the capacity for heat of different molecules is the 

 same. 



But the further a gas departs from the behaviour indicated by pv = ~Rd 

 the less nearly does it fall in with these laws. 



The following are some of Kegnault's results. The value for nitrogen 

 was calculated from those for air and oxygen : 



Simple Gases. 



