CONTRIBUTIONS TO MOLECULAR PHYSIOS 413 



is the great moderator of tlie earth's temperature, bring- 

 ing its extremes into proximity, and obviating contrasts 

 between day and nigbt which would render life insupport- 

 able. But we can advance beyond this general statement, 

 now that we know the radiation from aqueous vapor is 

 intercepted, in a special degree, by water, and, recipro- 

 cally, the radiation from water by aqueous vapor; for it 

 follows from this that the very act of nocturnal refrigera- 

 tion which produces the condensation of aqueous vapor 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 al- 

 lowing the violet and ultra-violet rays of the spectrum to 

 fall upon sulphate of quinine and other substances, Pro- 

 fessor Stokes has changed the periods of those rays. At- 

 tempts 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 with- 

 out 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; first, by its being in the opposite di- 

 rection — 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 aqueous vapor, before 

 their heat has assumed the radiant form. But it cannot 



