392 FKAGMENTS OF SCIENCE. 



now that we know the radiation from aqueous vapour 

 is intercepted, in a special degree, by water, and, re- 

 ciprocally, the radiation from water by aqueous va- 

 pour; for it follows from this that the very act of noc- 

 turnal 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 atmos- 

 phere and losing itself in space. 



And here we come to a question in molecular phys- 

 ics which at the present moment occupies attention. 

 By allowing the violet and ultra-violet rays of the spec- 

 trum to fall upon sulphate of quinine and other sub- 

 stances 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 believ- 

 ing, 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 oppo- 

 site direction that is, from a lower refrangibility to a 

 higher; and secondly, in the circumstance that the 

 lime is heated by the collision of the molecules of aque- 

 ous 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 ether could be rendered 

 .sufficiently intense.* The effect in principle is the 



* This was soon afterwards accomplished. See pp. 48, 49. 



