320 Rain Falling at Rothamsted 



which, as we have seen, is probably a minimum value for the total 

 brought down in the rain. 



There is a further difficulty in the assumption that atmospheric 

 pollution is the main source of the ammoniacal nitrogen of country air. 

 Atmospheric pollution of cities is as bad in winter as in summer if not 

 worse, so that if it were the source of most of the ammonia in country 

 rain we should expect at least as much in winter as in summer: in 

 jjoint of fact there is less. And, moreover, so far as Rothamsted is 

 concerned, the only town of any size to the south-west is Reading, but 

 as this is more than forty miles away, it is hardly likely to serve as a 

 sufficient reservoir. 



The formation of nitrates or nitrous fumes in the air is commonly 

 attributed to the major electrical discharges, — thunderstorms, etc. 

 Berthelot has shown that the silent electric discharge causes a production 

 of nitric acid from moist nitrogen and oxygen^. There is also the 

 possibility that the electrical stresses in the atmosphere may have some 

 effect; Chree"^ estimates these at about 200 to 300 volts per metre at 

 ground level, and much more at higher levels: at the top of tall trees 

 there may be 5000 volts per metre. The potential gradient required to 

 make a spark pass in ordinary air at normal pressure is of the order of 

 30,000 volts per centimetre. 



It has further been suggested by Soddy^ that nitrous oxide may be 

 formed by the action of the radium emanations always present in the 

 lower portions of the atmosphere. 



There are, however, other possible sources. Dust invariably contains 

 nitrates, and in summer the atmosphere contains more dust than in the 

 winter: the rain as we have seen also contains more nitrate. Gas flames 

 and fires also produce nitrous fumes. 



The close relationship between the amounts of ammoniacal nitrogen 

 and of nitric nitrogen in the rain throws important light on the origin of 

 the nitric compound. It must either be formed from the ammonia or 

 come from the same source. It is possible, but not easy, to conceive 

 of nitric oxides or acid compounds coming from the sea ; it is not difficult 

 to conceive of such compounds coming from the soil and from the air of 

 cities. Formation from ammonia would present no serious difficulty: 

 nitric compounds might arise from the oxidation of ammonia under the 

 influence of the minor electrical disturbances and electrical stresses in the 



> Compt. Rend., 1906, 142, 1367. 



^ Journ. Roy. Meleorol. Soc, 1915, 41, 121. 



« Chemical Soc. Re.jmrls, 1911, 8, 299. 



