48 THE NATUKE AND [SECT. n. 



comparing the specific heats of bodies, that of water at 60 is considered to be 

 unity, and therefore becomes a measure of all the rest. 1 



69. The property of bodies to hold different quantities of heat at the same 

 temperature, is sometimes called CAPACITY FOR HEAT ; but this term should be applied 

 only to the whole quantity of heat in a body, otherwise it becomes the same as 

 specific heat. Hence, when I speak of the capacity of a body for heat, it must be 

 understood as applied to the whole quantity of heat the body contains. 



70. The dimensions of bodies are enlarged when heat is poured into them, 

 and they contract when it is taken from them : and the natural consequence of 

 bodies absorbing different quantities of heat to cause an equal change of tempera- 

 ture, is, that they do not all expand nor contract alike by the change. 



The incontrovertible fact, that different substances have different capacities for 

 heat, being established, another necessarily presents itself, which, though it could 

 not possibly escape observation, has seldom been properly applied : it is, that in 

 everj 7 chemical change we effect, we are altering the capacities of bodies for heat, 

 and, consequently, deranging the equilibrium of heat ; for the products differ in 

 their capacity from the ingredients. 



71. By the mere addition of heat many solids assume the form of liquids, and 

 liquids the gaseous state. On the other hand, gases, by an abstraction of heat, 

 become liquids, and liquids solids. But even this change of state is accompanied 

 by a change of capacity. The capacity of steam for heat is greater than that of 

 water ; for steam requires an additional quantity of heat ; and that heat which is 

 required to expand the particles of a liquid to the distance they are apart in the 

 state of steam, does not affect the thermometer ; that is, when a given quantity of 

 water, after being heated to 212, is converted into steam of the same temperature, 

 the heat necessary to produce the entire change, from water to steam, would raise 

 the temperature of nearly six times as much water from the mean temperature 

 to 212. 



72. The heat absorbed by steam or vapour during its formation, is called 

 LATENT HEAT : it is, however, a term which conveys a false notion of the state of heat 

 in bodies ; for the heat is not latent, it is simply a difference of quantity, and not 

 of quality ; and some term that would convey a more accurate idea of the pheno- 

 mena would be better. 



1 The specific heat of a body is the quantity of heat requisite to change its temperature any 

 stated number of degrees, compared with that which would produce the same effect on water at 

 60 ; and it is therefore expressed by the fraction 



quantity of heat to change the temperature by any given amount, say 1 

 quantity of heat to change an equal weight of water at 60 by the same amount. 

 An equal volume of water may be taken instead of an equal weight. In any case, therefore, it is 

 necessary to understand whether the term ' specific heat ' is applied to volumes or weights. 



