234 LIGHT. 



tion. The theory subsequently received a great extension from the ingenious 

 labors of Fresnel ; and the still more recent researches of Arago, Poisson, 

 Herschel. Airy, and others, have conferred on it so great a degree of proba- 

 bility, that it may almost be regarded as ranking in the class of demonstrated 

 truths. " It is a theory," says Herschel, " which, if not founded in nature, is 

 certainly one of the happiest fictions that the genius of man has yet invented 

 to group together natural phenomena, as well as the most, fortunate in the sup- 

 port it has received from whole classes of new phenomena, which at their 

 discovery Deemed in irreconcilable opposition to it. It is, in fact, in all its 

 applications and details, one succession of felicities ; inasmuch as that we may 

 almost be induced to say, if it be not true, it deserves to be." 



Light and heat are so intimately related to each other, that philosophers 

 have doubted whether they are identical principles, or merely coexistent in 

 the luminous rays. They possess numerous properties in common : being 

 reflected, refracted, and polarized, according to the same optical laws, and even 

 exhibit the same phenomena of interference. Most substances during combus- 

 tion give out both light and heat ; and all bodies, except the gases, when heated 

 to a high temperature, become incandescent. Nevertheless, there are many 

 circumstances in which they appear to differ. 



A thin plate of transparent glass interposed between the face and a blazing 

 fire intercepts no sensible portion of the light, but most sensibly diminishes 

 the heat. Light and heat are therefore not intercepted alike by the same sub- 

 stances. Heat is also combined in different degrees with the different rays of 

 the solar spectrum. A very remarkable discovery on this subject was made 

 by Sir William Herschel, which would seem to establish the independence of 

 the heating and illuminating effects of the solar rays. Having placed ther- 

 mometers in the several prismatic colors of the solar spectrum, he found the 

 heating power of the rays gradually increased from the violet (where it was 

 least) to the extreme red, and that the maximum temperature existed sonu dis- 

 tance beyond the red, out of the visible pail of the spectrum. The experiment 

 was soon after repeated with great care by Berard, who confirmed Herschel's 

 conclusions relative to the augmentation of the calorific power from the violet 

 to the red, and not beyond the spectrum. This discovery of the inequality of 

 the heating power of the different rays led to the inquiry whether the chemical 

 action produced by light upon certain bodies was merely the effect of the heat 

 accompanying it, or owing to some other cause. By a series of delicate ex- 

 periments, Berard found that this action is not only independent of the heating 

 power, but follows entirely a different law : its intensity being greater in the 

 violet ray, where the heating power is the least, and least in the red ray, where 

 the heating power is the greatest. We are thus led to the conclusion that the 

 solar rays possess at least three distinct powers those of heating, illumina- 

 ting, and effecting chemical combinations and decompositions ; and these pow- 

 ers are distributed among the different refrangible rays in such a manner as to 

 show their complete independence of each other. 



I shall dismiss this subject, however, for the present, as I shall have another 

 opportunity of more fully developing the relations of heat and light. 



