216 POLARIZATION OF CALORIC. SECT. XXV. 



he inferred that heat from a terrestrial source is inca- 

 pable of being polarized. Professor Forbes of Edin- 

 burgh, who has recently prosecuted this subject with 

 great acuteness and success, came to the same conclu- 

 sion 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 ren- 

 dered the very small difference in the heat that was 

 transmitted in the two positions of the tourmalines im- 

 perceptible. The same conclusion had been come at 

 by M. Melloni ; nevertheless Mr. Forbes succeeded in 

 proving by numerous observations, that heat from vari- 

 ous sources was polarized by the tourmaline ; but that 

 the effect with non-luminous heat was very minute and 

 difficult to perceive, on account of the secondary radia- 

 tion. Though light is almost entirely excluded in one 

 position of the tourmalines, and transmitted in the other, 

 a vast quantity of radiant heat passes through them in 

 all positions. Eighty-four per cent, of the heat from an 

 argand lamp passed through the tourmalines in the case 

 where light was altogether stopped. It is only the dif- 

 ference in the quantity of transmitted heat that gives 

 evidence of its polarization. The second slice of tour- 

 maline, 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 accompany- 

 ing it under certain circumstances, as in reflection and 

 refraction, is capable of almost complete separation from 

 it under others. The separation has since been per- 

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

 through a combination of water and green glass, colored 

 by the oxide of copper. Even when the transmitted 

 light was concentrated 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 lam- 

 inae of mica, placed at the polarizing angle, and so cut 

 that the plane of incidence of the heat corresponded 

 with one of the optic axes of this mineral. The heat 

 transmitted through this apparatus was polarized when, 



