r 



RADIATION OF HEAT. 



411 



above the luminous spectrum, where the chemical influence is found, and C', 

 the lowest point in the green light, where its presence is discoverable. Let 



Fig. 2. 



us next suppose the screen M, N, to allow the luminous rays to be likewise 

 transmitted, these will be refracted by the prism, and will occupy the space L, 

 L/, corresponding to that already described as limited by the violet and red 

 lights. Finally, if the screen M, N, be removed, and all the rays allowed to 

 pass through the prism, the calorific rays will occupy the space from H, to H', 

 these being the points where the thermometer, in ascending and descending, 

 ceased to be affected. Thus, according to this supposition, three distinct 

 spectra, if they may be so called, are formed : the chemical spectrum, the lu- 

 minous spectrum, and tb.3 calorific spectrum. These spectra, to a certain ex- 

 tent, are superposed, or laid one upon another ; but the chemical spectrum ex- 

 tends beyond the luminous, at the upper part, while the calorific spectrum ex- 

 tends beyond the luminous, at the lower end. Each spectrum consists of rays 

 differently refrangible by the prism ; and if the middle ray be considered as 

 representing its mean refrangibility, it will follow that the mean refrangibility 

 of the chemical rays is greater than that of the luminous rays, and the mean 

 refrangibility of the luminous rays greater than that of the calorific rays. If 

 prisms of different materials be used, the relative degree of mean refrangibility 

 will be subject to change ; thus, the liquid prism above-mentioned, will cause 

 the mean refrangibility of the calorific rays to be more nearly equal to that of 

 the luminous rays than the glass prism. 



According to the other hypothesis, the solar beam consists of a number of 

 rays, which differ from each other in their capability of being deflected by any 

 refracting medium. When transmitted through a prism and received on a 

 screen, the most refrangible passes to the highest point, and the least refrangi- 

 ble to the lowest point, those of intermediate degrees of refrangibility taking 

 intermediate places. It is assumed that the rays which thus differ in refran- 

 gibility, have, also, different properties and qualities, and that they possess the 

 same quality in different degrees. Thus rays of different refrangibility have 

 different illuminating powers, and they possess the chemical agency with dif- 

 ferent degrees of energy. So far as the sensibility of thermometers enable us 

 to discover the existence of the calorific principle, it extends from a certain 

 point below R, to a certain point in the violet light, but the diminution of its 

 temperature is observed to be gradual in approaching its limit, and it is consis- 

 tent with analogy that it should exist, in a degree not discoverable by thermom- 

 eters, beyond these points. Although, therefore, the thermometer does not in- 

 dicate the calorific principle in the invisible chemical rays at the top of the 

 spectrum, yet we cannot infer that these rays are altogether destitute of that 

 principle, without assuming that the sensibility of thermometers has no limiis. 

 In like manner the chemical influence, so far as experiment determines its 

 presence, ends somewhere in the green light, about the middle of the luminous 



