MOTIVE POWER OF HEAT. 105 



The law of MM. Clement and Desormes re- 

 ferred to on page 92 gives ns this datum. The 

 constituent heat of vapor of water being always the 

 same at any temperature at which vaporization 

 takes place, if 550 degrees of heat are required to 

 vaporize water already brought up to 100 degrees, 

 550 -f- 100 or 650 will be required to vaporize the 

 same weight of water taken at zero. 



Making use of this datum and reasoning exactly 

 as we did for water at 100 degrees, we find, as is 

 easily seen, 



1.290 



for the motive power developed by 1000 units of 

 heat acting upon the vapor of water between one 

 degree and zero. This number approximates more 

 closely than the first to 



1.395. 



It differs from it only T *j, an error which does not 

 exceed probable limits, considering the great num- 

 ber of data of different sorts of which we have 

 been obliged to make use in order to arrive at this 

 approximation. Thus is our fundamental law veri- 

 fied in a special case.* 



* We find (Annales de Chimie et de Physique, July, 1818, 

 p. 294) in a memoir of M. Petit an estimate of the motive 

 power of heat applied to air and to vapor of water. This 



