RADIANT HEAT. 301 



ratnre ; if in contact — by conduction ; if at sensible distances — by 

 radiation, of the excess of heat ; and (in the latter case) whether the 

 radiation reach the cooler body- directly or by an intervening reflection. 



This appears sufficient to include the facts of Pictet's and Rum- 

 ford's experiments. 



7.) Alleged polarization of simple heat by reflection. 



Mens. J. E. Berard (Memoire sur les Proprietes des differentes 

 Especes de Rayons qu'on peut separer au moyen du Prisme de la 

 Lumiere solaire, Mem. de la Societe d' Arcueil, Paris, 1817, tome iii; 

 see also Annals of Phil, 0. S., ii, 1G4; Biot, Trade de Phys., iv,) 

 tried experiments for the polarization of heat. His apparatus was 

 the same as Mains' s, having the axis of revolution vertical ; but no 

 precautions of screening, &c, are mentioned. He used an air ther- 

 mometer containing a bubble of alcohol in the tube, in the focus of a 

 reflector, moving round along with the second glass: a ball of copper 

 about two inches in diameter was in the focus of a reflector placed 

 in the position for polarization of light. (His experiments on heat 

 with light will be referred to in another place.) He tried the effect 

 with the metal heated below luminosity, and assured himself that 

 there was a difference in the degree of heat reflected in the two rec- 

 tangular azimuths of the second glass. 



I have attempted to repeat these experiments with the same kind 

 of apparatus, carefully screened and arranged with the tube horizon- 

 tal ; but could produce no diminution in the proper position. — (Edinb. 

 Journal of Science, N. S., vol. x, p. 207.) 



I also tried the experiment with a delicate mercurial thermometer, 

 comparing this case with others, (referred to in their proper place,) 

 in which light accompanied the heat ; but in the former could detect 

 no difference in a long series of repetitions. 



The total effect is in all cases extremely small, and the disturbing 

 causes considerable, especially the heating of the glasses, &c. The 

 whole experiment was very unsatisfactory. — (Edinb. Journal of Sci- 

 ence, N. S., vol. vi, p. 297.) 



c.) Effect of lite nature of surfaces on the emission of simple heat. 



1.) Count Rumford (Nicholson's Journal, ix, 60) employed two simi- 

 lar vessels of hot water of the same temperature — one naked, the 

 other coated with linen, glue, black or white paint, or smoked with 

 a candle ; the results were, 



Naked vessel cooled 10 degrees in 55 minutes. 

 Coated " * 10 " 36| 



Mr. Murray supposes a relation between radiating and conducting 

 powers. — (System of Chem., i, 326,334. See Phil. Trans., 1804, p. 

 90, &c.) 



2.) The most complete investigation of this and other parts of the 

 subject has been made by Professor Leslie in his Inquiry into the Na- 

 ture and Propagation of Heed, 1804. 



He first used hot water in a globe of tin, in which the inserted 

 thermometer fell a given quantity, with the tin bright, in 156 minutes; 

 with the tin coated with lampblack, in 81 minutes. 



