48 MOTIVE POWER OF HEAT. 



Wherever there exists a difference of tempera- 

 ture, wherever it has been possible for the equilib- 

 rium of the caloric to be re-established,, it is possible 

 to have also the production of impelling power. 

 Steam is a means of realizing this power, but it is 

 not the only one. All substances in nature can 

 be employed for this purpose,, all are susceptible of 

 changes of volume, of successive contractions and 

 dilatations, through the alternation of heat and cold. 

 All are capable of overcoming in their changes of 

 volume certain resistances, and of thus developing 

 the impelling power. A solid body a metallic 

 bar for example alternately heated and cooled in- 

 creases and diminishes in length, and can move 

 bodies fastened to its ends. A liquid alternately 

 heated and cooled increases and diminishes in vol- 

 ume, and can overcome obstacles of greater or less 

 size, opposed to its dilatation. An aeriform fluid is 

 susceptible of considerable change of volume by 

 variations of temperature. If it is enclosed in an 

 expansible space, such as a cylinder provided with 

 a piston, it will produce movements of great ex- 

 tent. Vapors of all substances capable of passing 

 into a gaseous condition, as of alcohol, of mercury, 

 of sulphur, etc., may fulfil the same office as vapor 

 of water. The latter, alternately heated and 

 cooled, would produce motive power in the shape 



