200 LIGHT, CALORIC, F.LECTRICITY AND 31 AG.VETISM . 



those substances which tiansmit light or are transparent, mostly trans- 

 mit heat also or are transcalent, although some transparent bodies are 

 scarcely transcalent, and vice versa ; transparency and transcalency being 

 rarely proportionate to each other. A similar parallelism is also found 

 to exist in reference to refraction and polarization. When it is remem- 

 bered that a great difl'erence in these respects exists between heat of dif- 

 ferent intensities, just as there is between light of different colors, and 

 that the higher the temperature or greater the intensity of the heat the 

 more nearly does it approach light in all its laws, the probability of a 

 community of nature between them, or of their having a common ori- 

 gin becomes almost a certainty. 



But this common cause or agent, to which both are referable, may be 

 so .modified in in action as to present but one class of the eflects at the 

 same time. The lunar ray, for instance, which is a reflection of that of 

 the sun, comes to us almost totally deprived of every trace of heat, this, 

 in ordinary language, being absorbed by the surface of the moon upon 

 which it falls ; and some animal and vegetable substances in a certain stage 

 of decay emit phosphorescent light, whose heat is inappreciable, just aa 

 the light from bodies of low temperature is incapable oi' being observed. 

 In the solar spectrum there is likewise a partial separation of the light 

 and the heat. 



2. Between light, heat, and electricity, there is also a mutual and in- 

 timate relationship. In every case of the passage of free electricity 

 from point to point, as in the discharge of an electrical jar or a flash of 

 lightning, light and heat are both evolved. These three are either so 

 many manifested effects of a common agent, or two of them the product 

 of the third, unless it be supposed that, owing to a strong and mutual 

 affinity, they accompany each other. That the light and heat are not 

 due to the sudden condensation of the air by the electricity in its pas- 

 sage, is proved by the fact that they are alike evolved when it passes 

 through a vacuum, as shovvn in the experiment with a long exhausted 

 tube, or by the Auroral streamers which have their existence, to say the 

 least, in very highly rarified air. In the Voltaic arrangement these 

 three agents seem to be evolved simultaneously or give rise the one to 

 the other ; for charcoal may be ignited, even in vaccuo, emitting heat 

 the most intense, and light emulating the sun in brilliancy. Besides 

 these, other points of approximation might, if necessary, be cited. 



3. But the most intimate relationship is found to exist between elec- 

 tricity and magnetism. They are perfectly reciprocal, the one, with 

 equal ease, giving rise to the development of the other. Many of their 

 laws are almost perfectly convertible. In both, bodies similarly charged 



