410 



RADIATION OF HEAT. 



which was observed to increase in ascending toward the upper extremity, 

 might not exist in the space above that point, where no luminous rays were 

 apparent. They accordingly found, on exposing substances highly susceptible 

 of this chemical influence in the several spaces occupying the upper part of the 

 spectrum, and also in the space immediately above V, that the chemical action 

 was continued, as they had anticipated, beyond the luminous rays ; and as the 

 maximum heating-power, was found below R, so the maximum chemical influ- 

 ence was found to be in the space above V, in ascending beyond that point the 

 chemical influence rapidly diminished until it disappeared. It follows, there- 

 fore, that there are a number of chemical rays proceeding from the prism more 

 refrangible than any luminous rays, and falling on the screen above the point 

 V, in the space C. These chemical rays are found to be altogether destitute 

 of the heating principle, or at least, their effects on a thermometer were inap- 

 preciable. 



The experiments of Herschel were repeated by several other philosophers, 

 with various success, some being unable to detect any calorific rays beyond 

 luminous spectrum, others detecting their existence, but fixing the maximum 

 calorific influence in the red rays, and others again agreeing in all respects 

 with Herschel. Of these, the most valuable were experiments instituted by 

 Berard, in the laboratory of Berthollet at Paris. This philosopher used a 

 heliostat, which is an instrument constructed for the purpose of reflecting a 

 ray of the sun constantly in one direction, notwithstanding the change of posi- 

 tion of the sun by its diurnal motion.-- He thus obtained a perfectly steady and 

 immoveable spectrum ; and he repeated the experiment under much more fa- 

 vorable circumstances than those in which Herschel's investigations were con- 

 ducted. 



These experiments fully corroborated the results of former investigations, 

 and put beyond all question the presence of invisible rays beyond both 

 extremities of the spectrum, the one possessing the chemical, the other the 

 calorific property. Berard, however, found the maximum calorific influence 

 exactly at the extremity of the luminous spectrum, where the bulb of a 

 thermometer was completely covered with red light. The only difference then 

 which remained to be accounted for in the results of different experiments, was 

 the point of maximum calorific power, and it was conjectured by Biot that this 

 apparent discordance might be accounted for by the different materials of 

 which the prisms were composed. This conjecture was subsequently verified 

 by Seebeck, who proved that the position of greatest calorific intensity de- 

 pended on the nature of the prism by which the rays are refracted. He found 

 that a hollow prism, filled with water or alcohol, fixed the point of greatest 

 calorific intensity in the yellow rays. If filled with a solution of corrosive 

 sublimate, or with sulphuric acid, this point was found in the orange ray. 

 When a prism of crown-glass was used, it was situated in the red ray, but 

 when a prism of flint-glass was used, the point of greatest calorific intensity 

 took the position which Herschel assigned to it, in the non-luminous space 

 below the red ray. Thus all the apparent discordances in the experiment 

 were satisfactorily accounted for. The results of these experiments have 

 given rise to two distinct hypotheses respecting the constitution of solar light. 



In one it is supposed that the solar ray, S, S, is composed of three distinct 

 physical principles : the chemical, the luminous, and the calorific. Let us 

 imagine a screen, M, N, fig. 2, placed between the prism and window-shutter, 

 which is capable of intercepting the luminous and the calorific principle, but 

 which allows the chemical rays to be transmitted. In that case, the prism 

 will refract the chemical rays, and cause them to diverge and occupy a space 

 on the screen between the point C, and C', corresponding to the highest point 



