298 vnA GMENTS OF SCIENCE. 



C. to 3,259 degrees C., we must conclude that the aug- 

 mentation of temperature is applied to .an increase of 

 amplitude or width of swing, and not to the introduc- 

 tion of quicker periods into the radiation. 



The part played by aqueous vapor in the economy of 

 nature is far more wonderful than has been hitherto sup- 

 posed. To nourish the vegetation of the earth the actinic 

 and luminous rays of the sun must penetrate our atmos- 

 phere; and to such rays aqueous vapor is eminently trans- 

 parent. The violet and the ultra-violet rays pass through 

 it with freedom. To protect vegetation from destructive 

 chills the terrestrial rays must be checked in their transit 

 toward stellar space; and this is accomplished by the 

 aqueous vapor diffused through the air. This substance is 

 the great moderator of the earth's temperature, bringing 

 its extremes into proximity, and obviating contrasts between 

 day and night which would render life insupportable. 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, reciprocally, the radi- 

 ation from water by aqueous vapor; for it follows from this 

 that the very act of nocturnal refrigeration 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 

 allowing 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. 

 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 with- 

 out success. Such a change of period, I agree with Dr. 

 Miller in believing, occurs when the limelight is produced 

 by an oxyhydrogen 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 direc- 

 tion that is, from a lower refrangibility to a higher; and, 



