226 PROPAGATION OF HEAT. SECT. XXV. 



possible that they should be in absolute repose ; so that, 

 as far as our knowledge extends, motion seems to be a 

 law of matter. 



Heat applied to the surface of a fluid is propagated 

 downward very slowly, the warmer and consequently 

 lighter strata always remaining at the top. This is the 

 reason why the water at the bottom of lakes fed from 

 alpine chains is so cold ; for the heat of the sun is trans- 

 fused but a little way below the surface. "When heat 

 is applied below a liquid, the particles continually rise 

 as they become specifically lighteir in consequence of 

 the caloric, and diffuse it through the mass, their place 

 being perpetually supplied by those that are more dense. 

 The power of conducting heat varies materially in dif- 

 ferent 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 ah* 

 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 dis- 

 charged 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 thermom- 

 eter, caloric pervades bodies in an imperceptible or latent 

 state ; and their capacity for heat is so various, that veiy 

 different quantities of caloric are required to raise differ- 

 ent substances to the same sensible temperature ; it is 

 therefore evident that much of the caloric is absorbed, 

 or becomes latent and insensible 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 vapor. 

 When a solid is converted into a liquid, a greater quan- 

 tity 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 



