110 MOTIVE POWER OF HEAT. 



on solids and liquids a very great strain in order to 

 produce in them a reduction of volume comparable 

 to that which they experience in cooling (cooling 

 from 100 to zero, for example). Now the cooling 

 requires a greater abstraction of caloric than would 

 simple reduction of volume. If this reduction 

 were produced by mechanical means, the heat set 

 free would not then be able to make the tempera- 

 ture of the body vary as many degrees as the cool- 

 ing makes it vary. It would, however, necessitate 

 the employment of a force undoubtedly very con- 

 siderable. 



Since solid bodies are susceptible of little change 

 of temperature through changes of volume, and 

 since the condition of the most .effective employ- 

 ment of heat for the development of motive power 

 is precisely that all change of temperature should be 

 due to a change of volume, solid bodies appear but 



111 fitted to realize this power. 



The same remarks apply to liquids. The same 

 reasons may be given for rejecting them.* 



We are not speaking now of practical difficulties. 



* The recent experiments of M. Oerstedt on the com- 

 pressibility of water have shown that, for a pressure of 

 five atmospheres, the temperature of this liquid exhibits 

 no appreciable change. (See Annales de Ohimie et de 

 Physique, Feb. 1823, p. 192.) 



