548 HISTORY OF THERMOTICS. 



firmed by M. Melloni. Doubts were suggested whe- 

 ther the different effect in the opposite positions 

 might not be due to other circumstances; but Pro- 

 fessor 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 ana- 

 logous 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 re- 

 fraction. We are naturally much tempted to put 

 the same interpretation upon the depolarizing effect 

 in the case of heat ; but perhaps the assertion of 

 the analogy between light and heat to this extent 

 is 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 



