MOTIVE POWER OF HEAT. 123 



the vapor of water is its excessive tension at an 

 elevated temperature ; now this defect exists still 

 more strongly in the vapor of alcohol. As to the 

 relative advantage in a greater production of mo- 

 tive power, an advantage attributed to it, we 

 know by the principles above demonstrated that it 

 is imaginary. 



It is thus upon the use of atmospheric air and 

 vapor of water that subsequent attempts to perfect 

 heat-engines should be based. It is to utilize by 

 means of these agents the greatest possible falls of 

 caloric that all efforts should be directed. 



Finally, we will show how far we are from having 

 realized, by any means at present known, all the 

 motive power of combustibles. 



One kilogram of carbon burnt in the calorimeter 

 furnishes a quantity of heat capable of raising one 

 degree Centigrade about 7000 kilograms of water, 

 that is, it furnishes 7000 units of heat according to 

 the definition of these units given on page 100. 



The greatest fall of caloric attainable is measured 

 by the difference between the temperature pro- 

 duced by combustion and that of the refrigerant 

 bodies. It is difficult to perceive any other limits 

 to the temperature of combustion than those in 

 which the combination between oxygen and the 

 combustible may take place. Let us assume, how- 



