140 Professor Forbes on the Reflection 



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. Melloni had 

 published a second memoir, which, after many of my experiments 

 were made, I met with in the fifty-fifth volume of the Annates 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. Mellon i 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 circum- 

 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. Melloni'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 



