MOTIVE POWER OF HEAT. 83 



The elevation of temperature ought, evidently, 

 to be still more considerable if the capacity of the 

 air for heat becomes less as its volume diminishes. 

 Now this is probable, and it also seems to follow 

 from the experiments of MM. Delaroche and 

 Berard on the specific heat of air taken at different 

 densities. (See the Memoire in the Annales de 

 Chimie, t. Ixxxv. pp. 72, 224.) 



The two theorems explained on pp. 72 and 81 

 suffice for the comparison of the quantities of heat 

 absorbed or set free in the changes of volume of 

 elastic fluids, whatever may be the density and the 

 chemical nature of these fluids, provided always 



to (Hf) a and the temperature should rise another degree. 

 After x similar reductions the volume becomes (HI) 37 ' an d 

 the temperature should be raised x degrees. If we suppose 

 ({{l) x T^, and if we take the logarithms of both, we find 



x - about 300. 

 If we suppose (Hf) x = i> we find 



ar=80; 

 which shows that air compressed one half rises 80. 



All this is subject to the hypothesis that the specific heat 

 of air does not change, although the volume diminishes. 

 But if, for the reasons hereafter given (pp. 86, 89), we re- 

 gard the specific heat of air compressed one half as 

 reduced in the relation of 700 to 616, the number 80 must 

 be multiplied by |ff, which raises it to 90. 



