SECT, xxv.] POLARIZATION OF CALORIC. 251 



a plane perpendicular to the polarized ray, the light is 

 stopped when the plane containing the optic axis of the mica 

 is parallel or perpendicular to the plane of polarization. 

 Now, instead of light, if heat from a non-luminous source be 

 polarized in the manner described, it ought to be transmitted 

 and stopped by the interposed mica under the same circum- 

 stances under which polarized light would be transmitted 

 or stopped. Professor 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 polarization of heat. It therefore follows, that if 

 heat were visible, under similar circumstances we should see 

 figures 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 conditions, 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 undulations of light. 



Since the power of penetrating glass increases in proportion 

 as the radiating caloric approaches the state 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 condition and properties of luminous caloric, 

 the invisible rays must at first be analogous to the least calo- 

 rific 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 glow-worm, and as no substance had hitherto 

 been found which instantaneously transmits radiant caloric 

 coming from a source of low temperature, it was concluded 



