Prof. Forbes on the Refraction and Polarization of Heat, 141 



that, when exposed simultaneously with the thermal pile to the 

 source of heat, the latter had almost assumed its maximum ef- 

 fect before the others had sensibly moved ; and it is obvious 

 that, in delicate experiments, where constancy in the produ- 

 cing cause is presumed, rapidity of execution is essential. In 

 short, with an air thermometer (which requires from 10 to 15 

 minutes to give a single result), the greater part of the expe- 

 riments to be described would have been impossible from this 

 cause alone, and the remainder would have been tedious be- 

 yond measure. It will therefore be conceived that were ther- 

 mometers enlarged so as to give as minute indications as the 

 multiplier, they would be utterly unmanageable. 



16. Of all the researches of M. Melloni on radiant heat 

 that of the refrangibility of non-luminous heat by a prism of 

 rock salt is the most striking. Viewing it in connexion 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 determinations of the stoppage of radiant heat, 

 by screens or media of different kinds. At the time when I 

 commenced these experiments, 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 Annales de Chimie. It appeared to 

 me a matter of great interest to determine the refrangibility of 

 non-luminous heat by direct experiment ; and, in doing this, 

 I was led to verify, in the fullest manner, the published expe- 

 riments of M. Melloni on the refraction of heat, not merely de- 

 rived 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 ihe prism which gives the maximum effect; and, 

 having given the angles made by the incident and emergent 

 rays with the sides of the prism under those circumstances, 

 we may compute the index of refraction for the rock-salt, in 

 regard to rays of heat. Upon making the calculation, it ap- 

 peared 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 heating rays. I afterwards found, upon reading M. 

 Melloni's second memoir, that he had experienced the same 

 difiiculties, and that, though he constructed a pile on purpose, 



