CONTRIBUTIONS TO MOLECULAR PHYSICS. 441 



that, beyond the limits of the red end of the solar spec- 

 trum, rays of high heating power exist which are incom- 

 petent to excite vision. The discovery is capable of 

 extension. Dissolving iodine in the bisulphide of car- 

 bon, a solution is obtained which entirely intercepts the 

 light of the most brilliant flames, while to the ultra-red 

 rays of such flames the same iodine is found to be per- 

 fectly diathermic. The transparent bisulphide, which 

 is highly pervious to invisible heat, exercises 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 intercepted, but the heat- 

 spectrum may be received upon a screen and there 

 examined. Falling upon a thermo-electric pile, its in- 

 visible 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 

 excited by waves of aether shorter and more quickly 

 recurrent 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 

 aethereal 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- 

 mous with accord, between the periods of the waves of 



