140 Professor Forses on the Refraction 
of the refrangibility of non-luminous heat by a prism of rock salt is 
the most striking. Viewing it in connection with the theory of 
heat, and its analogies with light, this experiment is even more 
important than those connected with the very obscure subject of 
absorption, which has been illustrated by his numerous determi- 
nations of the stoppage of radiant heat, by screens or media of 
different kinds. At the time when I commenced these experi- 
ments, in November last, I was not aware that M. Me.ioni had 
published a second memoir, which, after many of my experiments 
were made, I met with in the fifty-fifth volume of the Annales de 
Chimie. It appeared to me a matter of great interest to deter- 
mine the refrangibility of non-luminous heat by direct experi- 
ment; and, in doing this, I was led to verify, in the fullest man- 
ner, the published experiments of M. Mretxont on the refraction 
of heat, not merely derived from brass heated by an alcohol 
lamp, so as not to have the faintest luminosity in the dark, but 
also of heat derived simply from water under its boiling point. 
I found that so admirable was the sensibility of the instrument, 
that we may determine, with great accuracy, by repeated trials, 
the angular position of the prism which gives the maximum ef- 
fect; and, having given the angles made by the incident and 
emergent rays with the sides of the prism under those cireum- 
stances, we may compute the index of refraction for the rock- 
salt, in regard to rays of heat. Upon making the calculation, it 
appeared that the direction thus experimentally found, gave 
nearly the same result as for light, which was an ample proof of 
the reality and striking nature of the experimental result; but 
it at the same time appeared that the whole dispersion for the 
spectrum is so inconsiderable, that, in this way, we could hardly 
expect to obtain a numerical result for the dispersion of the heat- 
ing rays. I afterwards found, upon reading M. Me.tonr’s se- 
cond memoir, that he had experienced the same difficulties, and 
that, though he constructed a pile on purpose, he had not suc- 
ceeded in obtaining numerical results. He found, however, that 
