REPORT ON ELECTRICITY, MAGNETISM; AND HEAT. 23 
of the shies is requisite; in order that neighbouring bodies may 
assume the same temperature. The first assertion of this con¬ 
nexion appears to have exeited some surprise in Paris, yet it is 
easily demonstrable *. It was announced as a curious result of 
Fourier’s investigations, that if the law of the sines did not 
obtain in the radiation of heat from a surface, a particle would 
not necessarily assume the temperature of the inclosure in which 
it is contained:—that its temperature would depend upon its po¬ 
sition ; and within a shell of ice we should have at certain points 
the temperature of boiling water and of melting iron, arising from 
radiation alone. Perhaps this may become less apparently 
strange by attention to the following reasoning. The equili¬ 
brium and identity of temperature, between an including shell 
and an included body, cannot obtain upon the whole in every 
case, except it obtain between each pair of parts, taken on the 
surface of the body and of the shell respectively; that is, any part 
of the one surface, in its exchanges with any part of the other 
surface, must give and receive the same quantity of heat. Now 
the quantity exchanged, so far as it depends on the receiving 
surface, will, by geometry, be proportional to the sine of the 
obliquity of that surface; and as each surface may, in the ex¬ 
change, be considered as receiving, the quantity transferred 
must be proportional to the sines of the two obliquities, that is, 
to that of the giving as w r ell as the receiving surface. 
But though the law of the sines is thus manifestly true, w r e 
have still to ask what is the physical ground of it ? To this 
question also Fourier offers a reply. It arises, he says, from 
this : that the radiation takes place not from the surface alone 
of the body, but also from particles situated within a certain 
small depth of the surface. It is easy to see that on this sup¬ 
position a ray emitted obliquely from an internal particle will 
be less intense than one sent forth from the same particle per¬ 
pendicular to the surface, because the former will be inter¬ 
cepted in a greater degree, having a greater length of path 
within the body; and Fourier shows that whatever be the law 
of this intercepting power, the result will be, that the radiative 
intensity is as the sine of the angle made by the ray with the 
surface. 
Thus Fourier’s theory of molecular extra-radiation acquires, 
to say the least of it, great consistency. But this cannot be 
considered, I think, a sufficient ground for the hypothesis main¬ 
tained by Laplace and by M.Poisson, that conduction also takes 
place by intra-radiation , or that we must conceive bodies as 
composed of distant particles radiating upon each other. 
* Ann. Chim . iv. p. 129, 1817. 
