Kelal'ions of Air, Heat, and Cold. 449 



The mode by which heat is conducted through sub- 

 stances diflTerently constituted is various: in solid bodies it 

 is bv successive impressions : in fluids the mobility of the 

 particles aflectsthe mode of operation: the proximate portion 

 of the medium, dilating as it becomes warm, is gently forced 

 to recede; and being likewise rendered specifically lighter it 

 rises to the surface, diffused in horizontal strata, the hottest 

 particles occupying the highest part : hence heat descends 

 in fluids very tardily and with extreme difficulty; a circum- 

 stance which accounts for ihe great coldness of water at 

 tl)e bottom of deep lakes. 



" The increasing coldness of the water drawn up from 

 considerable depths in the ocean hass lately been proposed 

 as a sure mark of the approach of soundings, if not of ihe 

 land itself. 



" Since water, on bein<r heated, expands in a rapid pro- 

 gression, the portion of heat which it abstracts from a body 

 nnmersed in it, by means of the recession and incessant 

 change of its contiguous affected particles, must be greatly 

 augmented in the higher temperatures. Near the freezing 

 point, this influence becomes extremely small, and water 

 is there scarcely a better conductor than ice ; but, as it ap- 

 proaches to ebullition, it acquires such an increase of mo- 

 bility, as to conduct fieat five tinifs faster tlian m its torpid 

 state. In other liquids, the increase of temperature will 

 occasion a similar alteration of the conducting powers, 

 though not so marked, as their expansions deviate less from 

 an uniform progression. 



'• But, through air and other gaseous fluids, the con- 

 veyance of heat is still more complex; and a close investi- 

 gation of that process, by unfolding certain latent pro- 

 perties of matter, has led to some very unexpected and 

 interesting results. A new principle appears to combine 

 its influence, and the rale of dispersion, in aeriform media, 

 is found to depend chicfiv on the nature of the mere heated 

 surface. Frou) a polished metallic surface, heal is feebly 

 emitted ; but, from a surface of glass, or still better from 

 one of paper, it is discharged with profusion. If two equal 

 balls of thin bright silver, one of them entirely uncovercrl, 

 and the other sheathed in a case of cambric, be filled with 

 water slightly warmed, and then suspended in a close room, 

 the former will lose only il parts of its heal in the same 

 time that tfie latter will dissipate 10 parts. Of this ex- 

 penditure, lo parts fronj each of the balls is cominunicattd 

 ill the ordinary way, by the slow recession of the |)roxitnate 

 particles of air, as ihey come to be successively heated. 



Vol. 41. No. 182. June 1S13. F f Tlie 



