558 Principles of Weather Forecasting. 



plasm of living cells, whether this be in the root, stem or 

 leaf of plants or in the tissues of animals. In the develop- 

 ment of muscular and nervous energy large quantities are 

 used hy the animal kingdom, and other large volumes are 

 used by man with fuel as a source of power and heat. 



707. Nitrogen. The nitrogen of the atmosphere is pri- 

 marily the source of all nitrogen compounds of living 

 forms; and by its dilution of all the other ingredients it 

 modifies their physiological effects. 



708. Water. Moisture in the atmosphere greatly influ- 

 ences the temperature of the earth's surface, as it is very 

 opaque to dark heat waves radiated back into space. The 

 frosts forming under clear skies and the absence of them 

 when the air is damp are evidence of this influence. But 

 the chief function of water is found in its large movement 

 to the land in the form of rain and snow and its return 

 from the fields through springs and rivers to the seas. As 

 it falls it is food for plants and drink for animals, as it re- 

 turns it carries away soluble salts which, if left, would de- 

 velop sterile "alkali" lands. 



709. Dust. The dust particles give to the sky its blue 

 color and by their radiation of heat into space become cold 

 centers upon which moisture condenses and snow flakes 

 form. In this way they greatly influence the precipita- 

 tion, making it less violent than it might otherwise be. 



710. Carbon Dioxide. Carbon dioxide is the source of 

 all the carbon entering into the constitution of the tissues 

 of both plants and animals, and it is a constituent of the 

 great majority of feeding stuffs and of most organic com- 

 pounds. 



From recent investigations it is held that carbon dioxide 

 plays an important part, with water, in lessening the 

 transparency of the atmosphere to dark heat rays radiating 

 from the earth into space, and in this way holds our tern- 



