MOTIVE POWER OF HEAT. 75 



V -f- J T V are small relatively to the volumes 

 themselves, we may regard the quantities of heat 

 absorbed by the air in passing from the first of 

 these volumes to the second, and from the first to 

 the third, as sensibly proportional to the changes 

 of volume. We are then led to the establishment 

 of the following relation : 



The quantity of heat necessary to raise one de- 

 gree air under constant pressure is to the quantity 

 of heat necessary to raise one degree the same air 

 under constant volume, in the ratio of the numbers 



rhr + irh- to TIT; 



or, multiplying both by 116 X 267, in the ratio of 

 the numbers 267 + 116 to 267. 



This, then, is the ratio which exists between the 

 capacity of air for heat under constant pressure 

 and its capacity under constant volume. If the 

 first of these two capacities is expressed by unity, 

 the other will be expressed by the number 267 + 7 116 , 

 or very nearly 0.700; their difference, 1 0.700 or 

 0.300, will evidently express the quantity of heat 

 which will produce the increase of volume in the 

 air when it is heated one degree under constant 

 pressure. 



According to the law of MM. Gay-Lussac and 

 JDalton, this increase of volume would be the same 



