Proceedings of the Royal Society. 97 
It was shown that all these results follow from Prevost’s theory 
of exchanges. For if we suppose a plate of rock-salt placed in a 
chamber of lamp-black, all at 212°, then, since the temperature of 
the rock-salt remains the same, it must radiate as much as it ab- 
sorbs. But since it absorbs but a small proportion of the lamp- 
black heat, it will radiate but a small proportion, and since a thick 
plate of rock-salt would absorb more than a thin plate, it would also 
radiate more. 
The radiation of such a thin plate is therefore equal to its ab- 
sorption. 
It was then shown that for every separate ray of which the he- 
terogeneous radiation of 212° is composed this equality must hold ; 
and that for every such ray the absorption of such a thin plate = its 
radiation. 
It was shown that the reason why rock-salt is opaque to heat from 
rock-salt is this. There are a few rays out of the total lamp-black 
radiation of 212° for which rock-salt is opaque ; these rays, there- 
fore, are rapidly absorbed by a thin plate of rock-salt ; but the ra- 
diation being equal to the absorption for every kind of heat, this 
thin plate will chiefly radiate such rays, which will consequently be 
stopped by a screen of rock-salt. 
In conclusion, it was shown that if we have a chamber, whose 
walls are composed of different substances, kept at a uniform tem- 
perature, the heat radiated and reflected ^together from any given 
portion of the surface of its wall will be independent of the nature of 
the substance of which that surface is composed ; the only difference 
being, that in the case of a metal, it will be chiefly reflected and 
little radiated heat, while in the case of lamp-black it will be altoge- 
ther radiated heat. But for all substances, radiated -\- reflected 
heat = a constant quantity. 
The following Gentleman was elected an Ordinary Fel- 
low 
The Bev. Dr Stevenson. 
The following Gentleman was elected an Honorary Fel- 
low : — 
Professor A. D. Bache, Superintendent of the United States Coast 
Survey. 
VOL. iv. 
N 
