5*54 PASSAGE OF HEAT THROUGH FLUIDS. 



body form a continued ascending current, which carries the' 

 whole of the heat immediately towards the surface of the 

 liquid ; so that the strata of the liquid situated at a small dis- 

 tance under the hot body, are not sensibly heated by it. 

 A cold body When a solid body is plunged in a liquid which is hotter 



cools a liquid j^au the bodv, the particles of the liquid in contact with the 

 downwards and ,,,.', , , , i • ^ i i i • 



not upwanls. body bemg condensed by the cooling tncy undergo, descend in 



consequence of the increase of their spcciiic gravity, and fall to 

 the bottom of the liquid; and the strata situated above the 

 level of the cold body arc not cooled by it immediately. 

 The viscidity It is true, that the viscosity of liquids^ even of those which 



ventsfheirp'ar- Possess the highest known degree of fluidity, is still much too 

 tides from great to allow one of their particles individually being moved 

 moving sing y. ^^^^ ^^ -^.^ place by any change of specific gravity occasioned 

 by heat or cold ; yet this does not prevent currents from being 

 formed in the manner above described, by small masses of the 

 liquid composed of a great number of such particles. 

 Currents in The existence of currents in the ordinary cases of the heat- 



*"i^®- ing and cooling of liquids, cannot any longer be called in ques- 



tion ; but philosophers are not yet agreed with respect to the- 

 extent of the effects produced by those currents. 

 Conductorsand In treating of abstruse subjects, it is indispensably necessary 

 non-conduc- ^^ £^ ^^^j^j^ precision the exact meaning of the words we em- 

 ploy. The distinction established between conductors and 

 no/i'Conducfors o{ heat is too vague not to stand in need of 

 explanation. An example will shew the ambiguity of these 

 expressions. 

 Instance of ' If two equal cubes of any solid matter, copper for instance, 

 heat transmit- ^f j.^^^ inches in diameter, the one at the temperature 60°, the 

 other at 100°, be placed one above the other; the cold cube 

 will be heated by the hot one, and this last will be cooled. 

 _., ■ If the cold cube be placed upon a table,.and its upper sur- 



metallio i^late ; face covered by a large plate of metal, of silver for instance, 

 a quarter of an inch thick, and if the hot cube be placed upon 

 this plate, immediately above the cold cube, the heat will de- 

 scend through the metallic plate with a certain degree of facili- 

 ty, and will heat the cold cube. 



more slowly ^^ ^ dry board, of the same thickness with the metallic 



through a plate, be substituted in its place, the heat will descend through 



' the wood, but with much less celerity than through the plate 



of silver. 



But 



