CONTRIBUTIONS TO MOLECULAR PHYSICS. 389 



Sir William Herschel made the important discov- 

 ery that, beyond the limit? of the red end of the solar 

 spectrum, rays of high heating power exist which are 

 incompetent to excite vision. The discovery is capa- 

 ble of extension. Dissolving iodine in the bisulphide 

 of carbon, a solution is obtained which entirely inter- 

 cepts the light of the most brilliant flames, while to the 

 ultra-red rays of such flames the same iodine is found 

 to be perfectly diathermic. The transparent bisul- 

 phide, which is highly pervious to invisible heat, ex- 

 ercises on it the same absorption as the perfectly 

 opaque solution. A hollow prism filled with the opaque 

 liquid being placed in the path of the beam from an 

 electric lamp, the light-spectrum is completely inter- 

 cepted, but the heat-spectrum may be received upon a 

 screen and there examined. Falling upon a thermo- 

 electric pile, its invisible presence is shown by the 

 prompt deflection of even a coarse galvanometer. 



What, then, is the physical meaning of opacity and 

 transparency as regards light and radiant heat? The 

 visible rays of the spectrum differ from the invisible 

 ones simply in period. The sensation of light is ex- 

 cited by waves of ether shorter and more quickly re- 

 current than the non-visual waves which fall beyond 

 the extreme red. But why should iodine stop the 

 former and allow the latter to pass? The answer to 

 this question no doubt is, that the intercepted waves 

 are those whose periods of recurrence coincide with the 

 periods of oscillation possible to the atoms of the dis- 

 solved iodine. The elastic forces which keep these 

 atoms apart compel them to vibrate in definite periods, 

 and, when these periods synchronise with those of the 

 ethereal waves, the latter are absorbed. Briefly de- 

 fined, then, transparency in liquids, as well as in gases, 

 is synonymous with discord, while opacity is synony- 



