CHANGES OCCASIONED BY HEAT. 159 



accept the law of Dalton and Gay-Lussac, it follows that this result is 

 independent of any peculiar properties in the air employed ;, and thus 

 this measure has an additional character of generality and simplicity 

 which make it still more probable that it is the true standard. This 

 opinion is further supported by the attempts to include such facts in a 

 theory ; but before we can treat of such theories, we must speak of 

 some other doctrines which have been introduced. 



Sect. 2. Specific Heat. Change of Consilience. 



IN the attempts to obtain measures of heat, it was found that bodies 

 had different capacities for heat ; for the same quantity of heat, how- 

 ever measured, would raise, in different degrees, the temperature of 

 different substances. The notion of different capacities for heat was 

 thus introduced, and each body was thus assumed to have a specific 

 capacity for heat, according to the quantity of heat which it required 

 to raise it through a given scale of heat. 4 The term " capacity for 

 heat" was introduced by Dr. Irvine, a pupil of Dr. Black. For this 

 term, Wil eke, 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 experiments 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 therniotic facts is, that by the 

 sudden contraction of any mass, its temperature is increased. This is 

 peculiarly observable 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 affect- 

 ing many points of meteorology. The coefficient 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, 

 varying in density, the pressure is constant by which the air is con- 

 tained ; the other, when, varying in density, it is contained in a con- 

 stant space. 



A leading fact, also, with regard to the operation of heat on bodies 



See Cra-n-furd, On Heat, for the History of Specific Heat. 



