354 Proceedings of the British Association. 



that of torsion-balance cannot be applied, but of its extreme sen- 

 sibility and accuracy no doubt can be entertained. There are 

 also several very ingenious improvements in the mode of arran- 

 ging the indices and reading off from the scale, but these are in- 

 dependent of the peculiar and beautiful principle above describ- 

 ed, and could not be rendered intelligible but in detail. It is 

 to be hoped the author will give such details to the public. 



3. Professor Powell read an abstract on certain points con- 

 nected with the recent discoveries relative to radiant heat. 



The object of his communication was to state, that the author 

 felt particular satisfaction in observing, that M. Melloni (in his 

 second Memoir), describes a repetition of the expei-iment origi- 

 nally made by him, and recorded in the Phil. Trans, for 1825 

 ■with perfect success, by means of his extremely delicate appara- 

 tus. The confirmation is the more complete, as M. Melloni ap- 

 pears to lake up the inquiry with a different object. 



It is thus now established beyond question, that luminous 

 hot bodies are sending out two distinct sorts of heat, or two dis- 

 tinct heating agents at the same time, differing in their proper- 

 ties and mode of operation. 



Hence, the whole series of results of M. Melloni must be in- 

 terpreted with reference to this distinction ; and possibly the 

 consideration of it may remove some of the apparent anomalies. 



And further, if the distinction which he established in the 

 case of luminous hot bodies do really continue to exist (as it 

 seems it must do unless there be a breach of the law of conti- 

 nuity) at temperatures below that of vi.iible luminosity (itself a 

 very undefined point), then it ought to be rendered manifest by 

 a careful repetition of the same fundamental experiment, with 

 very delicate thermoscopes (either Melloni's or such as that used 

 by Dr Hudson), by which a smaller, though probably very mi- 

 nute difference of ratio, would be found in the effects on a black 

 and white thermoscopc, with and without a glass screen. 



Should this be found to be the case, in all cases of reflection, 

 polarization, &c. with non-luminous sources, it will henceforth 

 become necessary to distinguish to which of the two heating 

 causes they belong ; every analogy would lead us to suppose 

 they belong essentially to that of the two heating ag«its which 

 is so closely associated with the light, that it seems rather a pro- 



