324 Rain Falling at Rothamsted 



The anion III of chlorine in rainwater. 



The chlorine is determined in the rainwater at Rothamsted because 

 the values are wanted in connection with the drainage investigations. 

 The data are given in Tables 6 and 7 (pp. 335-6): they throw interesting 

 light on the origin of the rain. 



The average amount of chlorine in the Rothamsted rainwater is 

 2-3 parts per million, bringing down 16 lb. per acre per annum, but the 

 amount fluctuates considerably from year to year, the lowest being 

 10-3 lb. in 1889-90, and the highest 24-4 lb. in 1915-16. Most of 

 this is brought down during the months September to April: a much 

 smaller amount falls in the summer months. The fluctuations in the 

 amount of chlorine carried down per acre closely follow the fluctuations 

 in the amount of rainfall, especially in the months September to April : 

 during the summer months. May to August, the rain contains so little 

 chlorine that the fluctuations have less significance. This is well brought 

 out by the curves (Fig. 3); only in two periods 1896-1900 and 1904-8, is 

 there notable deviation from the close relationship in the winter months. 



It has been usual to attribute the chlorine to sea spray blown over 

 the land. This view is consistent with the facts, and would explain the 

 large amount of chlorine in winter rain and the small amount in summer 

 rain when gales are less common. 



Another source exists, however. Some of the chlorine may come from 

 fires: it is present in coal, which contains about 4 lb. per ton: it would 

 be liberated during combustion in the gaseous form or as volatile 

 chlorides. This source would be most in evidence during the winter. 



The change in composition of RothntiiMed rainwater. 



Looking over the figures for successive four year periods given in 

 Table 9 (p. 337) there seem to be signs of change in the composition of 

 rain. The chlorine is increasing in amount, so also is the nitrate; the 

 ammonia tends to fall, so that there is no steady alteration in the sum 

 of the ammoniacal and nitric nitrogen. It is more easy to suggest possible 

 explanations than to decide between them. The rainfall has increased 

 since 1908, and this tends to increase the quantity of substances brought 

 down per acre. The increase in rainfall has been especially marked in 

 the winter months: we have no wind records at Rothamsted, but it 

 is possible that the higher winter rainfall has been accompanied by 

 heavier gales carrying more chlorine from sea spray. The increase in 



