n 



09 ( 



RADIATION OF HEAT. 409 



out of their course these different-colored lights in different degrees. Thus it 

 is capable of deflecting the violet light more than the indigo, the indigo more 

 than the blue, and so on, each color in succession being more refrangible by the 

 prism than that which occupies a lower place, and red being therefore the 

 least refrangible component part of the solar beam. 



Let us now suppose that the bulbs of a series of thermometers are placed in 

 the different colored lights, from the violet to the red, in regular succession. 

 The relative heating-powers of these different colors will be indicated by the 

 effect which they produce on the several thermometers, the most powerful 

 being that which raises the thermometer exposed to its influence highest. 

 It is found that the thermometer whose bulb is covered with the violet light is 

 less elevated than that which is exposed to the indigo. This again is less 

 raised than that which is exposed to the blue, and the elevation of the several 

 thermometers go on, thus regularly increasing ; that which is acted upon by 

 the red light standing at a greater elevation than any of the others. Hence we 

 infer that the calorific power of the red light is greater than that of any other 

 component part of the solar beam. It might at first view be supposed that the 

 calorific power had some dependance on or connexion with the illuminating 

 power of light, and that the light which was most brilliant would likewise be 

 most hot. This, however, is not the fact ; for the most brilliant part of the 

 prismatic spectrum is found in the position of the yellow light, and the bril- 

 liancy gradually diminishes toward the extremity of the red, where the heat 

 is found to be greatest. 



It occurred to Sir William Herschel, that as hot bodies emit calorific rays 

 which are not luminous, it was possibla that non-luminous calorific rays might 

 exist in solar light itself. To determine this point, he placed a thermometer in 

 the space immediately below R, the red eSufemity of the spectrum. He accord- 

 ingly found, as he had anticipated, that the thermometer still continued to be 

 affected, and consequently that the presence of calorific rays, invisible and 

 non-luminous, was manifested ; but what was more singular, he found that the 

 calorific power of these invisible rays was even greater than that of the lumin- 

 ous red rays, in fact, the maximum effect of the calorific rays was found at a 

 point H, a little below R. From that point downward the calorific influence rap- 

 idly diminished, until it altogether disappeared. There are, therefore, a num- 

 ber of invisible rays proceeding from the prism, and occupying the space H, 

 below R. These rays are refracted by the prism in the same manner as the 

 luminous rays, but the refraction is less in quantity. These invisible rays also 

 differ from each other in refrangibility, in the same manner as the luminous 

 rays do, since they occupy a space of some extent below R. Those whose 

 position is lowest being less refrangible than those nearer to the luminous rays. 



Soon after these experiments of Sir William Herschel, the attention of 

 several distinguished philosophers was attracted to the investigation of the 

 properties of the prismatic spectrum, and among others the late Dr. Wollaston, 

 Ritter, and Beckmann. It had been long known that the solar light pro- 

 duced an influence on certain chemical processes. Thus the chloride of 

 silver, exposed to the direct rays of the sun, was known to acquire a black 

 color. Chemical effects were also produced on the oxides of certain metals. 

 It was shown by Scheele and others that these effects were produced by the 

 rays of light which occupy the upper part of the spectrum, and not at all by the 

 red rays. A feeble effect was produced by the green ray, and the chemical 

 energy was increased by ascending toward the violet ray. The circumstance 

 of Herschel having discovered invisible calorific rays under the lower extremity 

 of the spectrum, and even finding the point of extreme energy in that space, 

 suggested to these philosophers the inquiry, whether the chemical influence 



