296 FHAGMKNTS OP 



tude come into play in absorption and radiation atomic 

 complexity must also be taken into account. 1 would 

 recommend to the particular attention of chemists the 

 molecule of water; the deportment of this substance to- 

 ward radiant heat being perfectly anomalous, if the 

 chemical formula at present ascribed to it be correct. 



Sir William Herschel made the important discovery that, 

 beyond the limits of the red end of the solar spectrum, rays of 

 high heating power exist which are incompetent to excite 

 vision. The discovery is capable of extension. Dissolving 

 iodine in the bisulphide of carbon, 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 perfectly diathermic. The trans- 

 parent 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 ether 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 inter- 

 cepted waves are those whose periods of recurrence coincide 

 with the periods of oscillation possible to the atoms of the 

 dissolved iodine. The elastic forces which keep these 

 atoms apart compel them to vibrate in definite periods, 

 and, when these periods synchronize with those of the 

 ethereal waves, the latter are absorbed. Briefly defined, 

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

 synonymous with discord, while opacity is synonymous 

 with accord, between the periods of the waves of ether and 

 those of the molecules on which they impinge. 



According to this view transparent and colorless sub- 

 stances owe their transparency to the dissonance existing 



