72 MOTIVE POWER OF HEAT. 



absorbed by the gas in its expansion of volume, or 

 that which this gas relinquishes during compres- 

 sion. We are led, then, to establish the following 

 proposition : 



When a gas passes without change of tempera- 

 ture from one definite volume and pressure to an- 

 other volume and another pressure equally definite, 

 the quantity of caloric absorbed or relinquished is 

 always the same, ivhatever may be the nature of 

 the gas chosen as the subject of the experiment. 



Take, for example, 1 litre of air at the tempera- 

 ture of 100 and under the pressure of one atmos- 

 phere. If we double the volume of this air and 

 wish to maintain it at the temperature of 100, a 

 certain quantity of heat must be supplied to it. 

 Now this quantity will be precisely the same if, 

 instead of operating on the air, we operate upon 

 carbonic-acid gas, upon nitrogen, upon hydrogen, 

 upon vapor of water or of alcohol, that is, if we 

 double the volume of 1 litre of these gases taken at 

 the temperature of 100 and under atmospheric 

 pressure. 



It will be the same thing in the inverse sense if, 

 Instead of doubling the volume of gas, we reduce 

 it one half by compression. The quantity of heat 

 that the elastic fluids set free or absorb in their 

 changes of volume has never been measured by 



