THE ATMOSPHERE 81 



down by the rain on each acre. To these must be added 

 about 0*8 lb. of nitrogen as organic matter, giving a total of 

 about 4*5 lb. of nitrogen per acre per annum. 



As the mean of investigations carried on at seven Conti- 

 nental stations between 1864 and 1872, 0*47 part of nitrogen 

 as nitric acid and 1*26 parts of ammonia per million of rain 

 were found, equivalent to a total of 10*18 lb. of combined 

 nitrogen per acre per annum. These results are much higher 

 than the English figures, probably because some of them were 

 obtained in or near towns. In tropical countries the amounts 

 are generally much higher. Thus in Pretoria in the year 

 July 1, 1904, to June 30, 1905, the writer found a mean of 

 1-194 parts per million of nitrogen as ammonia and 0*196 part 

 per million of nitrogen as nitric acid in the rain, equivalent, 

 with a total rainfall of 24*31 inches, to 767 lb. of combined 

 nitrogen per acre per annum. It is also noteworthy that in 

 the Transvaal the whole of this is brought down during the 

 active growing season — the summer, September to April — for 

 the winter months are almost absolutely rainless. 



The combined nitrogen of the air in the rain is of consider- 

 able importance to plants. The amount given above for 

 Rothamsted, 4-5 lb. combined nitrogen per acre per annum, 

 is equivalent to the application of about 27 lb. of nitrate of 

 soda, while that for Pretoria corresponds to 47*6 lb. of nitrate 

 of soda, or 36*2 lb. of sulphate of ammonia. 



Ozone is an active form of oxygen obtained from ordinary 

 oxygen by the action of the electric discharge, especially the 

 form known as the silent discharge, or by the slow action of 

 certain readily oxidisable substances — e.g., phosphorus — upon 

 oxygen or air It is a gas with a peculiar odour, capable of 

 oxidising most organic substances and many metals. It is 

 doubtful if its presence in air has any beneficial effect upon 

 people breathing it, but it affords a proof that the air is free 

 from oxidisable organic matter, and probably from micro- 

 organisms. Its amount in the air is veiy variable, but nlways 



