NITRIC ACID IN EAIN. 33 



This gives a total of combined nitrogen in the rain of 3'74B>. 

 per acre per annum. 



The mean amount from seven Continental agricultural sta- 

 tions between 1864 and 1872 is 0'47 parts per million of nitro- 

 gen as nitric acid and l-26pts. per million of ammonia in the 

 rain, yielding a total fall of 1O18R). of combined nitrogen per 

 acre. 



Results obtained in New Zealand and in Japan agree better 

 with the Rothamsted results. 



On the other hand, Miintz and Marcano* in 1883-1885, as 

 the result of over 120 analyses of rain-water in Venezuela, 

 found as a mean 2'23pts. of nitric acid per million (equal to 

 0'578 parts of nitrogen), the maximum amount being 16-25 

 (4*2 parts of nitrogen) and the minimum O2pts. ( 4 05 nitrogen) 

 per million. In the island of Reunion an average of 2'67pts. 

 per million of nitric acid (equal to O69 parts of nitrogen) was 

 found. As the rainfall, too, is much greater, it is obvious that 

 the amount of nitric nitrogen conveyed to the soil by the rain 

 is very much greater in the tropics than in England. 



The same authors in 18901 found a mean of l'55mgm. of 

 ammonia ( = l'28mgm. of nitrogen) per litre in rain-water 

 collected in the tropics. Here, too, the amount is much 

 greater than in England. 



Ozone, the active form of oxygen, is present in air, but in 

 very varying and always excessively small amount. The 

 measurement of the absolute proportion of ozone in air is 

 difficult, and the greater number of observations recorded 

 merely give the relative amounts according to an arbitrary 

 scale (Schonbein and Houzeau). Moreover it is very probable 

 that many of the results obtained are really due, wholly or in 

 part, to the presence of hydrogen peroxide. 



Houzeau, who estimated the relative amount of ozone 

 present in air by the colour imparted to strips of red litmus 

 paper coated for half their length with 1 % solution of potas- 

 sium iodide and exposed for a given time, concluded that the 

 amount of ozone in country air was, at the most, 1 in 450,000 

 by weight or 1 in 700,000 by volume. He observed that the 



* Compt. Rend. 108, 1062 ; J.C.8. 1889, abst. 923. t Compt, Rend. 114, 184. 



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