392 FKAGMENTS OF SCIENCE. 



in a special degree, by water, and, reciprocally, the radi- 

 ation from water by aqueous vapour ; for it follows from 

 this that the very act of nocturnal refrigeration which 

 produces the condensation of aqueous vapour at the 

 surface of the earth giving, as it were, a varnish of 

 water to that surface imparts to terrestrial radiation 

 that particular character which disqualifies it from 

 passing through the earth's atmosphere and losing itself 

 in space. 



And here we come to a question in molecular physics 

 which at the present moment occupies attention. By 

 allowing the violet and ultra-violet rays of the spectrum 

 to fall upon sulphate of quinine and other substances 

 Professor Stokes has changed the periods of those rays. 

 Attempts have been made to produce a similar result at 

 the other end of the spectrum to convert the ultra-red 

 periods into periods competent to excite vision but 

 hitherto without success. Such a change of period, I 

 agree with Dr. Miller in believing, occurs when the lime- 

 light is produced by an oxy-hydrogen flame. In this 

 common experiment there is an actual breaking up of long 

 periods into short ones a true rendering of unvisual 

 periods visual. The change of refrangibility here effected 

 differs from that of Professor Stokes; firstly, by its 

 being in the opposite direction that is, from a lower 

 refrangibility to a higher ; and, secondly, in the circum- 

 stance that the lime is heated by the collision of the mole- 

 cules of aqueous vapour, before their heat has assumed 

 the radiant form. But it cannot be doubted that the 

 same effect would be produced by radiant heat of the same 

 periods, provided the motion of the aether could be ren- 

 dered sufficiently intense. 1 The effect in principle is the 

 same, whether we consider the lime to be struck by 



1 This was soon afterwards accomplished. See pp. 48. 49. 



