SECT, xxv.] POLAKIZATION OF CALORIC. 249 



from a terrestrial source is incapable of being polarized. 

 Professor Forbes of Edinburgh, who has recently 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 tourmalines imperceptible. The 

 same conclusion had been come at by M. Melloni ; neverthe- 

 less Mr. Forbes succeeded in proving, by numerous observa- 

 tions, that heat from various 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 radiation. Though light is almost entirely ex- 

 cluded 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 difference 

 in the quantity of transmitted heat that gives evidence of its 

 polarization. The second slice of tourmaline, when per- 

 pendicular 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 con- 

 cluded 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 separa- 

 tion 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 

 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 laminae of 

 mica, placed at the polarizing angle, and so cut that the plane 

 of incidence of the heat corresponded with one of the optic 



