CONTRIBUTIONS TO MOLECULAR PHYSICS. 391 



the radiation from the hydrogen flame. Thus we es- 

 tablish accord between the periods of the atoms of cold 

 water and those of aqueous vapour at a temperature 

 of 3259 C. But the periods of water have already 

 been proved to be ultra-red hence those of the hydro- 

 gen flame must be sensibly ultra-red also. The ab- 

 sorption by dry air of the heat emitted by a platinum 

 spiral raised to incandescence by electricity is insensi- 

 ble, while that by the ordinary undried air is 6 per 

 cent. Substituting for the platinum spiral a hydro- 

 gen flame, the absorption by dry air still remains in- 

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

 cent, of the entire radiation. The temperature of the 

 hydrogen flame is, as stated, 3259 C.; that of the aque- 

 ous vapour of the air 20 C. Suppose, then, the tem- 

 perature of aqueous vapour to rise from 20 C. to 

 3259 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 vapour in the economy 

 of nature is far more wonderful than has been hitherto 

 supposed. To nourish the vegetation of the earth 

 the actinic and luminous rays of the sun must pene- 

 trate our atmosphere; and to such rays aqueous vapour 

 is eminently transparent. 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 towards stellar space; 

 and this is accomplished by the aqueous vapour dif- 

 fused through the air. This substance is the great 

 moderator of the earth's temperature, bringing its ex- 

 tremes into proximity, and obviating contrasts between 

 day and night which would render life insupportable. 

 But we can advance beyond this general statement, 



