96 FRAGMENTS OF SCIENCE. 
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-spectrurn 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 tire 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 
