CONTRIBUTIONS TO MOLECULAR PHYSICS 413 



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

 ing its extremes into proximity, and obviating contrasts 

 between day and night 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 



