ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 61 



soil, or upon the foliage of a cultivated field, they bring 

 down to the reach of vegetation in a given time a quantity 

 of ammonia, far greater tlum what is diifiised throughout 

 the limited volume of air which contributes to the nour- 

 ishment of plants. Tlie solubility of carbonate of ammo- 

 nia in water has already been mentioned. In a rain-fall 

 Ave have the atmosphere actually washed to a great de- 

 gree of its ammonia, so tliat nearly the entire quantity of 

 this substance which exists between the clouds and the 

 earth, or in that mass of atmosphere through Avhich the 

 rain passes, is gathered by the latter and accumulated 

 within a small space. 



Proportion of Ammonia in Rain-water, etc. — The pro- 

 portion of ammonia * which the atmospheric waters thus 

 collect and bring down upon the surface of the soil, or 

 upon the foliage of plants, has been the subject of inves- 

 tigations by Boussingault, Bineau, Way, Knop, Bobiere, 

 and Bretschneider. The general i-esiilt of their accordant 

 investigations is as follows : In rain-water the quantity of 

 ammonia in the entiiic fall is very variable, ranging in the 

 country from 1 to 33 parts in 10 million. In cities the 

 amount is larger, tenfold the above quantities having been 

 observed. 



The first portions of rain that fall usually contain much 

 more ammonia than the latter portions, for the reason that 

 a certain amount of water suffices to wash the air, and 

 what rain subsequently falls only dilutes the solution at 

 first formed. In a long-continued rain, the water that 

 finally falls is almost devoid of ammonia. In rains of 

 short duration, as well as in dews and fogs, which occasion- 

 ally are so heavy as to admit of collecting to a sufficient 

 extent for analysis, the proportion of ammonia is greatest, 

 and is the greater the longer the time that has elapsed 

 since a previous precipitation of water. 



* III all (luaiilitative statemonis roj^'ardiui,' ammonia, NH3 is to be miderstoodi 

 ftnauotNH^O. 



