172 NOTICES OF THE MEETINGS [April 23, 
WEEKLY EVENING MEETING, 
Friday, April 23. 
W. Pousz, Esa. M.A., F.R.S., Treasurer and Vice-President, 
in the Chair. 
Tue Rev. Bapen Powe tt, M.A., F.R.S., &c. 
SAVILIAN PROFESSOR OF GEOMETRY, OXFORD, 
On the Analogies of Light and Heat. 
Tue 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 
absorptive 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 recognized by Sir J. 
Leslie) and shewed 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 duminous hot bodies it does so to an extent suf- 
ficient to give very marked indications. But this (as the author 
shewed, in the experiments referred to), applies to that portion only 
of the compound rays, which is also transmissible through glass, 
the non-transmissible portion is subject wholly to the former con- 
dition, as are all the rays from non-luminous sources (as was shewn 
by Leslie and others). 
