Royal Institution. 535 



in the whole circuit be equivalent to that of a copper wire of about 100 

 feet long and about one-eighth of an inch in diameter, and if the engine 

 be allowed to move at such a rate as by inductive reaction to diminish 

 the strength of the current to the half of what it is when the engine 

 is at rest, would produce mechanical effect at the rate of about one- 

 fifth of a horse-power. The electromotive force of a copper and bis- 

 muth element, with its two junctions at 0° and 1°, being found by 

 Pouillet to be about y^ of the electromotive force when the junc- 

 tions are at 0° and 100°, must be about 163. The value of 9 for 

 copper and bismuth, according to these results (and to the value 

 160" 16 of fx at 0°), or the quantity of heat absorbed in a second of 

 time by a current of unit strength in passing from bismuth to copper, 

 when the temperature is kept at 0°, is j^, or very nearly equal to 

 the quantity required to raise the temperature of a grain of water 

 from 0° to 1° Cent. 



ROYAL INSTITUTION OF GREAT BRITAIN. 



April 23, 1852. — On the Analogies of Light and Heat. By the 

 Rev. Baden Powell, M.A., F.R.S., &c, Savilian Professor of Geo- 

 metry, Oxford. 



The researches of Sir W. Herschel, Sir J. Leslie, M. De la Roche, 

 and others, long since established the existence of well-marked dif- 

 ferences in character, not only between the radiation from the sun 

 and that from terrestrial sources, but even among these latter, 

 according as the source was luminous or not ; and this especially as 

 regarded its transmissibility through various screens and the absorp- 

 tive effect of different surfaces. 



But the most striking peculiarity in the radiation from flame was 

 established by Sir W. Herschel and afterwards extended to gas-lights 

 by Mr. Brande, in that, even at considerable distances, after passing 

 through a thick glass lens, without heating it, the concentrated rays 

 produced heat on a blackened thermometer at the focus, exactly as in 

 the case of the solar rays. 



This pointed to a peculiar distinction (also recognised by Sir J. 

 Leslie), and showed that the mere proportion of heat transmitted by 

 a screen (as in De la Roche's experiments) was not the essential 

 characteristic, but that further distinction as to the specific nature of 

 the rays, was wanted. This want it was attempted in some measure 

 to supply in some experiments by the author of this paper (Phil. 

 Trans. 1825), in which the character of the different rays as to trans- 

 missibility through screens was examined in combination with the 

 conditions of the absorbing surface. 



This last is a point even yet little understood ; but thus much is 

 clear : — 



(1) A certain peculiarity of texture in the external lamina is 

 favourable to the absorption of radiant heat, probably in all cases. 



(2) Darkness of colour is peculiarly favourable to the effect for the 

 sun's rays, and wholly overrules the first condition. 



In terrestrial luminous hot bodies it does so to an extent sufficient 

 to give very marked indications. But this (as the author showed, 



