SECT. XXVII. POLARIZATION OF HEAT. 267 



a non-luminous source be polarized in the manner described, it 

 ought to be transmitted and stopped by the interposed mica 

 under the same circumstances under which polarized light would 

 be transmitted or stopped. Professor Forbes 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 213, 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 longer than the undulations of light ; and 

 it 4 has already been mentioned that Professor Draper considers 

 them to be normal, like those of sound. 



That light and heat are both vibrations of the ethereal medium 

 is not the less true on account of the rays of heat being unseen, 

 for the condition of visibility or invisibility may only depend 

 upon the construction of our eyes, and not upon the nature of 

 the motion which produces these sensations in us. The sense of 

 seeing may be confined within certain limits. The chemical 

 rays beyond the violet end of the spectrum may be too rapid, or 

 not sufficiently excursive, in their vibrations, to be visible to the 

 human eye ; and the calorific rays beyond the other end of the 

 spectrum may not be sufficiently rapid, or too extensive, in their 

 undulations, to affect our optic nerves, though both may be 

 visible to certain animals or insects. We are altogether ignorant 

 of the perceptions which direct the carrier-pigeon to his home, 

 or of those in the antennse of insects which warn them of the 

 approach of danger ; nor can we understand the telescopic vision 

 which directs the vulture to his prey before he himself is visible 

 even as a speck in the heavens. So, likewise, beings may 

 exist on earth, in the air, or in the waters, which hear sounds 

 our ears are incapable of hearing, and which see rays of light 

 and heat of which we are unconscious. Our perceptions and 

 faculties are limited to a very small portion of that immense 

 chain of existence which extends from the Creator to evanescence. 



The identity of action under similar circumstances is one of 



K 2 



