218 POLARIZATION OF CALORIC. SECT. XXV. 



nous source be polarized in the manner described, it 

 ought td be transmitted and stopped by the interposed 

 mica under the same circumstances under which polar- 

 ized light would be transmitted or stopped. Prolessor 

 Forbes has found that this is really the case, whether he 

 employed heat from luminous or non-luminous sources : 

 and he had evidence also of circular and elliptical polar- 

 ization of heat. It therefore follows that if heat were 

 visible, under similar circumstances we should see fig- 

 ures perfectly similar to those given in Note 207, and 

 those following; and as these figures are formed by the 

 interference of undulations of light, it may be inferred 

 that heat, like light, is propagated by undulations of the 

 ethereal medium, which interfere under certain condi- 

 tions, and produce figures analogous to those of light. 

 It appears also from Mr. Forbes's experiments, that the 

 undulations of heat are probably longer than the undu- 

 lations of light. 



Since the power of penetrating glass increases in pro- 

 portion as the radiating caloric approaches the slate of 

 light, it seemed to indicate that the same principle takes 

 the form of light or heat according to the modification 

 it receives, and that the hot rays are only invisible light; 

 and light, luminous caloric. It was natural to infer, that 

 in the gradual approach of invisible caloric to the condi- 

 tion and properties of luminous caloric, the invisible 

 rays must at first be analogous to the least calorific part 

 of the spectrum, which is at the violet extremity an 

 analogy which appeared to be greater, by all flame 

 being at first violet or blue, and only becoming white 

 when it has attained its greatest intensity. Thus, as 

 diaphanous bodies transmit light with the same facility 

 whether proceeding from the sun or from a glowworm, 

 and as no substance had hitherto been found which in- 

 stantaneously transmits radiant caloric coming from a 

 source of low temperature, it was concluded that no 

 such substance exists, and the great difference between 

 the transmission of light and radiant heat was thus re- 

 ferred to the nature of the agent of heat, and not to the 

 action of matter upon the calorific rays. M. Melloni, 

 however, has discovered in rock-salt a substance which 

 transmits radiant heat with the same facility whether it 



