30 ELEMENTARY AGRICULTURAL CHEMISTRY 



The nitric acid is probably present as ammonium nitrate (or a 

 portion of that reported as nitric acid may really be nitrous 

 acid as ammonium nitrite). The source of the ammonia is 

 probably the putrefaction of nitrogenous organic matter, as 

 the amount is generally larger near towns than in the open 

 country. 



The quantities of these substances are generally so small 

 that they can only with great difficulty be estimated, and 

 since they are readily soluble in water they are largely re- 

 moved from the air by rain. In rain-water, therefore, they 

 become more concentrated, and analyses of rain are of much 

 interest as indicating the quantities of these and other sub- 

 stances present in the air. Many analyses have been pub- 

 lished — e.g., those made by Angus Smith in 1872, of which 

 the following is an abstract : the figures represent parts per 

 million of the rain water. 



Experiments at Rothamsted in 1880-89 showed a mean of 

 0-426 part of nitrogen as ammonia and 0*139 part of nitrogen 

 as nitrates per million of rain, which, with a total annual 

 rainfall of 29'27 inches, gave a total of 2*823 lb. of nitrogen 

 as ammonia and 1)17 lb. of nitrogen as nitrates brought 



