260 LATENT HEAT. [SKCT. xxv. 



that are more dense. The power of conducting heat varies 

 materially in different liquids. Mercury conducts twice as 

 fast as an equal bulk of water, which is the reason why it 

 appears to be so cold. A hot body diffuses its caloric in the 

 air by a double process. The air in contact with it being 

 heated, and becoming lighter, ascends and scatters its caloric, 

 while at the same time another portion is discharged in 

 straight lines by the radiating powers of the surface. Hence 

 a substance cools more rapidly in air than in vacuo, because 

 in the latter case the process is carried on by radiation alone. 

 It is probable that the earth, having originally been of very 

 high temperature, has become cooler by radiation only. The 

 ethereal medium must be too rare to carry off much caloric. 

 Besides the degree of heat indicated by the thermometer, 

 caloric pervades bodies in an imperceptible or latent state ; 

 and their capacity for heat is so various, that very different 

 quantities of caloric.are required to raise different substances 

 to the same sensible temperature ; it is therefore evident that 

 much of the caloric is absorbed, or becomes latent and in- 

 sensible to the thermometer. The portion of caloric requisite 

 to raise a body to a given temperature is its specific heat ; 

 but latent heat is that portion of caloric which is employed 

 in changing the state of bodies from solid to liquid, and from 

 liquid to vapour. When a solid is converted into a liquid, a 

 greater quantity of caloric enters into it than can be detected 

 by the thermometer ; this accession of caloric does not make 

 the body warmer, though it converts it into a liquid, and is 

 the principal cause of its fluidity. Ice remains at the tem- 

 perature of 32 of Fahrenheit till it has combined with or 

 absorbed 140 of caloric, and then it melts, but without 

 raising the temperature of the water above 32 ; so that 

 water is a compound of ice and caloric. On the contrary, 

 when a liquid is converted into a solid, a quantity of caloric 

 leaves it without any diminution of temperature. Water at 

 the temperature of 32 must part with 140 of caloric before 

 it freezes. The slowness with which water freezes, or ice 



