318 Rain FaUltuj at Rotltamsted 



the ammonia may be expected to dissolve freely, in which case constant 

 renewal would be necessary in order to account for the fact that high 

 rainfall brings down more ammonia than low rainfall. 



This constant renewal necessitates contact with a source of ammonia 

 which, moreover, gives ujj its ammonia uniformly so that a definite 

 equilibrium is attained. As soon as a shower of rain has fallen and removed 

 the ammonia from the atmosphere a further sujsply must be drawn 

 from the source sufficient to restore the disturbed equilibrium. The 

 next shower removes some of this, but again the equilibrium is restored. 

 On this assumption the total quantity brought down by the rain in any 

 year would depend on the amount of the rainfall, but the quantity per 

 inch of rain would show less variation. 



Three possible sources have been considered: the sea, which has 

 already been discussed, the soil, and the atmosphere over large towns 

 and cities. 



The soil seems quite a likely source: changes are constantly occurring 

 there with formation of ammonia which is then transformed to nitrate. 

 The amount of ammoniacal nitrogen existing as such at any time is 

 at least 5 lb. per acre in the top 9 in., and its rate of diffusion into the 

 atmosphere might be fairly rapid. 



Hall and Miller^ endeavoured to obtain information as to this possi- 

 bility by exposing shallow dishes of sulphuric acid, some close to the 

 ground and others four feet above it, and then determining the ammonia 

 absorbed by the end of each month: the experiment lasted for two years. 

 The amounts absorbed corresponded only to 0-99 and 1-28 lb. per acre 

 in the respective years, — much less than those recorded by previous 

 observers, perhaps because of the efficiency with which dust and insects 

 were excluded: the lower dishes, however, did not usually contain more 

 ammonia than the upper ones, excepting when sulphate of ammonia 

 was applied to the soil. No conclusions could be drawn as to whether 

 the soil normally gives up ammonia to the atmosphere, or whether it 

 absorbs ammonia from the atmosphere. From the circumstance that 

 soil gives up ammonia when dressed with sulphate of ammonia it seems 

 legitimate to infer that some ammonia is continuously being evolved. 



The seasonal fluctuations in the amount of ammonia in the rain are 

 quite consistent with the view that the ammonia comes from the soil. 

 The amount of ammoniacal nitrogen in the rain is lowest in the four 

 months, January, February, March and April, when biochemical 



1 TUs Journal 1911,4, 46. 



