88 MOTIVE POWEH OF HEAT. 



Since the difference between the two capacities 

 for heat is constant, if one increases in arithmetical 

 progression the other should follow a similar pro- 

 gression: thus one law is applicable to specific 

 heats at constant pressure. 



We have tacitly assumed the increase of specific 

 heat with that of volume. This increase is indi- 

 cated by the experiments of MM. Delaroche and 

 Berard: in fact these physicists have found 0.967 

 for the specific heat of air under the pressure of 



the ratio between the specific heat at constant pressure and 

 the specific heat at constant volume varies very little with 

 the density of the gas. According to what we have just 

 seen, the difference should remain constant, and not the 

 ratio. As, further, the specific heat of gases for a given 

 weight varies very little with the density, it is evident that 

 the ratio itself experiences but slight changes. 



The ratio between the specific heat of atmospheric air at 

 constant pressure and at constant volume is, according 

 to MM. Gay-Lussac and Welter, 1.3748, a number almost 

 constant for all pressures, and even for all temperatures. 

 We have come, through other considerations, to the number 

 ^_11 6 = 1.44, which differs from the former ^, and we 

 have used this number to prepare a table of the specific 

 heats of gases at constant volume. So we need not regard 

 this table as very exact, any more than the table given on 

 p. 89. These tables are mainly intended to demonstrate 

 the laws governing specific heats of aeriform fluids. 



