410 FRAGMENTS OF SCIENCE 



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 spec- 

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

 petent to excite vision. The discovery is capable of ex- 

 tension. 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 dia- 

 thermic. The transparent bisulphide, which is highly 

 pervious to invisible heat, exercises on it the same ab- 

 sorption 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 com- 

 pletely intercepted, 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 vis- 

 ible 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 intercepted 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 



