SECT. XXVII. EXPERIMENTS OF PROF. FORBES. 265 



already shown that two slices of tourmaline, cut parallel to the 

 axis of the crystal, transmit a great portion of the incident light 

 when looked through with their axes parallel, and almost 

 entirely exclude it when they are perpendicular to one another. 

 Should radiant heat be capable of polarization, the quantity 

 transmitted by the slices of tourmaline in their former position 

 ought greatly to exceed that which passes through them in the 

 latter, yet M. Melloni found that the quantity of heat was the 

 same in both cases : whence he inferred that heat from a terres- 

 trial source is incapable of being polarized. Professor Forbes of 

 Edinburgh, who prosecuted this subject with great acuteness 

 and success, came to the same conclusion in the first instance ; 

 but it occurred to him, that, as the pieces of tourmaline became 

 heated by being very near the lamp, the secondary radiation 

 from them rendered the very small difference in the heat that 

 was transmitted in the two positions of the pieces of tourmaline 

 imperceptible. Nevertheless he succeeded in proving, by nume- 

 rous observations, that heat from various sources is polarized by 

 the tourmaline ; but that the effect with non-luminous heat is 

 very minute and difficult to perceive, on account of the secondary 

 radiation. Though light is almost entirely excluded in one posi- 

 tion of the pieces of tourmaline, and transmitted in the other, 

 a vast quantity of radiant heat passes through them in all posi- 

 tions. Eighty-four per cent, of the heat from an argand lamp 

 passed through them in the case where light was altogether 

 stopped. It is only the difference in the quantity of transmitted 

 heat that gives evidence of its polarization. The second slice of 

 tourmaline, when perpendicular to the first, stops all the light, 

 but transmits a great proportion of heat; alum, on the contrary, 

 stops almost all the heat, and transmits the light ; whence it 

 may be concluded that heat, though intimately partaking the 

 nature of light, and accompanying it under certain circumstances, 

 as in reflection and refraction, is capable of almost complete 

 separation from it under others. The separation has since been 

 perfectly effected by M. Melloni, by passing a beam of light 

 through a combination of water and green glass, coloured by the 

 oxide of copper. Even when the transmitted light was concen- 

 trated by lenses, so as to render it almost as brilliant as the 

 direct light of the sun, it showed no sensible heat. 

 Professor Forbes next employed two bundles; of laminse of 



N 



