470 ON IMPRESSIONS OF COLD 



rent of three miles an hour would accelerate tenfold the trans- 

 fer of heat. 



It isof essential importance, however, to remark, that in all 

 cases where the medium of communication is a liquid, the na- 

 ture of the hot surface, whether vitreous or metallic, roin»h or 

 polished, has no influence whatever in modifying the rate 

 with which the heat is drawn off and dispersed. Thus, 

 a hollow tin ball filled with hot water, will lose its heat just 

 as fast when immersed naked in cold water, as if it were cover- 

 ed with linen, or paper, or a coat of any sort of pigment. 



3. Heat is transferred through a gaseous medium by a more 

 complex process. It is partly conducted, as in the case of li- 

 quids, by successive communication through the stagnant mass, 

 joined to the more copious effect of the buoyant streaming of 

 the heated portions of the fluid. But another auxiliary prin- 

 ciple, depending merely on the nature of the heated surface, 

 now comes into action. Let two equal hollow balls of bright 

 thin silver, the one naked, and the other covered with a fold of 

 cambric or paper applied closely to its surface, be filled with 

 warm water, and suspended in a close room. The former 

 will be found to lose 11 parts of heat, in the same time that 

 the latter sheds 20 parts. Of the heat thus spent, 10 parts, 

 from each of the balls, are communicated in the ordinary 

 way, by the slow recession of the particles of air, as they 

 come to be successively heated by their proximity. The re- 

 maining portions of heat, consisting of 1 part from the bright 

 metallic surface, and 10 parts from the cambric, are pro- 

 pagated through the aerial medium, by some peculiar pro- 

 cess. In like manner, two equal hollow balls of glass, the one 

 gilt with gold or silver leaf, being filled with warm water, and 

 suspended in a room, the naked one will discharge 13 parts 

 of heat, while the gilt one will lose only 7 parts ; 6 parts be- 

 ing spent by each, in the same way as if they had been im- 

 mersed 



