AIR 



6 9 



Since nitrogenous animal and vegetable matters are con- 



stantly undergoing decay, some ammonia is always pres- 



ent in the air. The ammonia gas unites with the carbon 



dioxid and forms ammonium carbonate. In barns and 



stables, where the ventilation 



is poor, abnormal amounts of 



carbon dioxid and ammonia 



are formed from the respiration 



and waste products of animals. 



When carbon dioxid forms to 



such an extent that it produces 



a white coating upon stones 



and boards, it shows enough 



ammonium carbonate to be in- 



jurious to animals. Nitrogen, 



in traces, in the form of ammo- 



nium nitrate and nitrite, is 



also present in the air. The 



amount of combined nitroge- 



nous compounds in the air is 



small and is not sufficient to 



furnish food for plants. 



70. rioisture. The amount 

 of moisture in the air ranges 

 between wide limits, from com- 



plete saturation to desert conditions. When the air con- 

 tains all the moisture it can hold, it is said to be saturated. 

 Under ordinary conditions, the humidity, or per cent, of 

 saturation ranges between 60 and 85. The amount of 

 moisture in the air has an~_*influence upon plant growth, 



