CHANGES OCCASIONED BY HEAT. 553 



The term "capacity for heat" was introduced by 

 Dr. Irvine, a pupil of Dr. Black. For this term, 

 Wilcke, the Swedish physicist, substituted " specific 

 heat;" in analogy with "specific gravity." 



It was found, also, that the capacity of the same 

 substance was different in the same substance at 

 different temperatures. It appears from experi- 

 ments of MM. Dulong and Petit, that, in general, 

 the capacity of liquids and solids increases as we 

 ascend in the scale of temperature. 



But one of the most important thermotic facts 

 is, that by the sudden contraction of any mass, its 

 temperature is increased. This is peculiarly observ- 

 able in gases, as, for example, common air. The 

 amount of the increase of temperature by sudden 

 condensation, or of the cold produced by sudden 

 rarefaction, is an important datum, determining the 

 velocity of sound, as we have already seen, and 

 affecting many points of meteorology. The coeffi- 

 cient which enters the calculation in the former 

 case depends on the ratio of two specific heats of 

 air under different conditions; one belonging to it 

 when the pressure is constant by which the air is 

 contained; the other, when it is contained in a 

 constant space. 



A leading fact, also, with regard to the opera- 

 tion of heat on bodies is, that it changes their form, 

 as it is often called, that is, their condition as solid, 

 liquid, or air. Since the term "form" is employed 

 in too many and various senses to be immediately 

 understood when it is intended to convey this pe- 



