34: PRESSURE OF GASES. 



will be practically the same as the simpler ratio, 



Vo Q <f 



5. In the case where the specific heats are the same, or very 

 nearly so, which is that of powders having a nitrate as base, and 

 a certain number of other explosive substances, this ratio 

 reduces itself to 



6. In other cases, if it be supposed with some mathematicians 

 that the specific heat of a compound is equal in theory to the 

 sum of those of its elements, the ratio of the specific heats 



c' a' 



- might be replaced by the ratio * of the number of the atoms, 



c q 



that is, of the elementary units of the compound (each of these 

 units being referred to its atomic weight), or 



4 



But this formula is very open to dispute, owing to the inac- 

 curacy of the hypothesis relative to the specific heats (see p. 19). 

 On this point, we will only state that the specific heat of a 

 molecule of potassium sulphate would be according to theory l 

 equal to 24 x 7 = 16*8, while experiments have given, even 

 near the ordinary temperature, 3 3 '2, that is, the double. It 

 would be easy to give very numerous examples of the same 

 kind, derived from the study of solid and liquid compounds. 



7. Owing to these disagreements, it is preferable to take for c 

 and c' their experimental values, and to admit that their ratio 

 remains nearly constant, notwithstanding the doubts which are 

 connected with the application of these values to very high 

 temperatures. 



The ratio of the characteristic products 



therefore only retains a purely empirical meaning, but it offers 

 the advantage of being calculable for the unit weight, from 

 simply experimental data, and without introducing any hypo- 

 thesis relative to the laws of gases. It furnishes the elements 

 of a first comparison between explosive substances, in the room 

 of a more perfect theory. 



1 2-4 is the specific heat at constant volume of the simple gases. 



