412 FRAGMENTS OF SCIENCE 



bon 0*07 of an incli in thickness transmits 72 per cent of 

 tlie incident radiation, and while every other liquid exam- 

 ined transmits more or less of the heat, a layer of water 

 of the above thickness is entirely opaque to the radiation 

 from the hydrogen flame. Thus we establish accord be- 

 tween the periods of the atoms of cold water and those 

 of aqueous vapor at a temperature of 3,259° C. But the 

 periods of water have already been proved to be ultra- 

 red — ^hence those of the hydrogen flame must be sensibly 

 ultra-red also. The absorption by dry air of the heat 

 emitted by a platinum spiral raised to incandescence by 

 electricity is insensible, while that by the ordinary undried 

 air is 6 per cent. Substituting for the platinum spiral a 

 hydrogen flame, the absorption by dry air still remains 

 insensible, while that of the undried air rises to 20 per 

 cent of the entire radiation. The temperature of the hy- 

 drogen flame is, as stated, 3,259° C. ; that of the aqueous 

 vapor of the air 20° C. Suppose, then, the temperature of 

 aqueous vapor to rise from 20° C. to 3,259° C, we must 

 conclude that the augmentation of temperature is applied 

 to an increase of amplitude or width of swing, and not to 

 the introduction 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 



