SBCT. XXV. NATURE OF CALORIC. 219 



originates in the brightest flame or lukewarm water, 

 and which consequently possesses the same permeability 

 with regard to heat that all diaphanous bodies have for 

 light. It follows, therefore, that the impermeability of 

 glass and other substances for radiant heat arises from 

 their action upon the calorific rays, and not from the 

 principle of caloric. But although this discovery changes 

 the received ideas drawn from M. de Laroche's experi- 

 ments, it establishes a new and unlooked-for analogy 

 between these two great agents of nature. True it is 

 that the separation of the luminous and calorific rays 

 shows that they must owe their immediate origin to two 

 different causes, at the same time it is quite possible 

 that these two causes themselves may be only different 

 effects of one single cause. The probability of light and 

 heat being modifications of the same principle is not 

 diminished by the calorific rays 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 agent 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 alto- 

 gether ignorant of the perceptions which direct the car- 

 rier-pigeon to his home, or of those in the antennae of 

 insects which warn them of the approach of danger; 

 nor can we understand the telescopic vision which di- 

 rects the vulture to his prey before he himself is visible 

 even as a speck in the heavens (N. 213). 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 exist- 

 ence which extends from the Creator to evanescence. 



The identity of action under similar circumstances is 

 one of the strongest arguments in favor of the common 



