252 NATURE OF CALORIC. [SECT. xxv. 



that no such substance exists, and the great difference bet ween 

 the transmission of light and radiant heat was thus referred 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 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 experiments, it establishes 

 a new and unlooked-for analogy between these two great 

 agents of nature. True it is that the separation of the lumi- 

 nous and calorific rays shows that they must owe their imme- 

 diate 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 di- 

 minished by the calorific rays being unseen ; for the condition 

 of visibility or invisibility may only depend upon the con- 

 struction 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 carrier- 

 pigeon to his home, or of those in the antennee of insects 

 which warn them of the approach of danger ; nor can we un- 

 derstand the telescopic vision which directs the vulture to his 

 prey before he himself is visible, even as a speck in the heavens 



