CONDUCTION' AND RADIATION. 155 



icflccted is so small that the inevitable errors might completely dis- 

 guise the whole difference in the two opposite positions. "\Vhcn Prof. 

 Forbes, of Edinburgh, (in 1834,) employed mica in the like experi- 

 ments, he found a very decided polarizing effect ; first, when the heat 

 was transmitted through several films of mica at a certain angle, and 

 afterwards, when it was reflected from them. In this case, he found 

 that with non-luminous heat, and even with the heat of water below 

 the boiling point, the difference of the heating power in the two 

 positions of opposite polarity (parallel and crossed*) was manifest. He 

 also detected by careful experiments, 29 the polarizing effect of tourma- 

 line. This important discovery was soon confirmed by M. Melloni. 

 Doubts were suggested whether the different effect in the opposite 

 positions might not be due to other circumstances', but Professor 

 Forbes easily showed that these suppositions were inadmissible ; and 

 the property of a difference of sides, which at first seemed so strange 

 when ascribed to the rays of light, also belongs, it seems to be proved, 

 to the rays of heat. Professor Forbes also found, by interposing a 

 plate of mica to intercept the ray of heat in an intermediate point, an 

 effect was produced in certain positions of the mica analogous to what 

 was called depolarization in the case of light ; namely, a partial 

 destruction of the differences which polarization establishes. 



Before this discovery, M. Melloni had already proved by experiment 

 that heat is refracted by transparent substances as light is. In the case 

 of light, the depolarizing effect was afterwards found to be really, as 

 we have seen, a dipolarizing effect, the ray being divided into two rays 

 by double refraction. "We are naturally much tempted to put the same 

 interpretation upon the dipolarizing effect in the case of heat ; but 

 perhaps the assertion of the analogy between light and heat to this 

 extent is as yet insecure. 



It is the more necessary to be cautious in our attempt to identify 

 the laws of light and heat, inasmuch as along with all the resemblances 

 of the two agents, there are very important differences. The power 

 of transmitting light, the diaphaneity of bodies, is very distinct from 

 their power of transmitting heat, which has been called diathermancy 

 by M. Melloui. Thus both a plate of alum and a plate of rock-salt 

 transmit nearly the whole light; but while the first stops nearly the 

 whole heat, the second stops very little of it ; and a plate of opake 



19 Ed. R. S. Transactions, vol. xiv. ; and Phil May. 1835, vol. v. p. 20?. Ib 

 vol. vii. p. 349. 



