CONTRIBUTIONS TO MOLECULAR PHYSICS. 391 



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 hydrogen flame must be 

 sensibly ultra-red also. The absorption by dry air of 

 the heat emitted by a platinum spiral raised to in- 

 candescence by electricity is insensible, while that by 

 the ordinary undried air is 6 per cent. Substituting 

 for the platinum spiral a hydrogen flame, the absorp- 

 tion by dry air still remains insensible, while that of 

 the undried air rises to 20 per cent, of the entire radia- 

 tion. The temperature of the hydrogen flame is, as 

 stated, 3259 C. ; that of the aqueous vapour of the 

 air 20 C. Suppose, then, the temperature of aqueous 

 vapour to rise from 20 C. to 3259 C., we must con- 

 clude that the yi lamentation of temperature is applied 

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

 to the introduction of quicker periods into the radia- 

 tion. 



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 penetrate 

 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 vegeta- 

 tion from destructive chills the terrestrial rays must be 

 checked in their transit towards stellar space ; and this 

 is accomplished by the aqueous vapour diffused through 

 the air. This substance is the great moderator of the 

 earth's temperature, bringing its extremes into prox- 

 imity, 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 vapour is intercepted, 

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