of Heat through Diathermal Bodies. 477 



Mr. Hudson experimented were by no means favourable for 

 studying the immediate transmission of radiant caloric through 

 solid bodies; and yet that philosopher ciles his results as facts 

 tending to prove that there is no immediate passage of simple 

 heat through that class of bodies. His induction, although 

 presented under a doubtful form, does not appear to me per- 

 missible. 



Mr. Powell performed in 1825 a very beautiful experiment 

 upon radiant caloric • ; it consists in proving that the ratio of 

 calorific absorption of a white surface to that of a black one 

 is not the same for the rays proceeding directly from the 

 source, and for the rays transmitted by a plate of glass. The 

 sources of heat employed by Mr. Powell were an Argand 

 lamp and iron heated to a bright red. I have had occasion 

 more recently to verify this fact, which holds good not only 

 with the glass, but with all diathermal substances, rock-salt 

 excepted. In order to explain this phasnomenon, as well as 

 the old experiments of calorific transmission, Mr. Powell ad- 

 mitted that flame and incandescent metals radiate two kinds 

 of heat, the luminous and the obscure, the first of which alone 

 is capable of traversing the glass, whilst the second is entirely 

 absorbed by that substance. He even now thinks that the 

 entire series of my experiments may be explained on this 

 supposition, which he without doubt has modified, in conced- 

 ing that the interception by solid bodies in general is not a 

 distinctive character of the non-luminous heat, since, in cer- 

 tain cases, it traverses these bodies with the same ease as the 

 most luminous heat. If Mr. Powell alludes to experiments 

 analogous to his own, that is to say, the series of observations 

 which have been made with the pile having one of its faces 

 whitened and the other blacked, I am of his opinion ; but I 

 differ from him totally if he admits that the hypothesis of two 

 heats suffices to explain all the facts relative to the transmis- 

 sion. I will limit myself to citing some results which appear 

 to me decisive. If we expose a common plate of glass of one or 

 two millimetres in thickness to the calorific rays of Locatelli's 

 lamp emerging from a black opake glass, then to the imme- 

 diate radiation of a plate of copper heated to 400° [Cent.?], 

 and finally to the heat emitted from a vessel full of boiling 

 water, we find that its transmission is T 7 G ^ to y 8 ^ of the inci- 

 dent heat in the first case, y 1 ^ to y 1 ^ in the second, and in 

 the third. Now here the three radiations consist exclusively 

 of non-luminous heat ; and yet their transmissibility across the 

 same plate is so different, that nearly all the incident rays 



[* See Phil. Trans. 1825? or Phil. Mag., First Series, vol. lxv. p. 437 ei 

 seq — Edit.] 



