28 On the Rain and Drainage - Waters at Rothamsted. 
The remainins constituents of drainage-water that we have to 
mention, namely the chlorides, ammonia, and nitrates, bring us 
to the point at which we can make use of the additional facts 
furnished by recent work at Rothamsted. As any detailed 
statements must be omitted for want of space, we can only 
consider the general facts which the results of the analyses 
teach. 
Chlorine is an element of very little agricultural importance. 
The wheat-crops in Broadbalk Field assimilate very little of the 
chlorides applied in the manure ; in the corn practically no 
chlorine is found ; in the straw only a small and variable quan- 
tity. Regarded simply as plant-food, chlorides might easily be 
dismissed from consideration. For our present purpose, how- 
ever, the chlorides have a special and very considerable im- 
portance. Chlorides and nitrates are both salts for which soil 
possesses apparently no chemical retentive power ; they are held 
by soil merely as in a sponge : their distribution in the soil is 
thus regulated by the amount of rain falling on the surface, and 
by the ordinary laws of diffusion. As the amount of chlorine 
applied to each plot in the manure is with a single exception 
(Plot 2) fairly well known, the proportion of chlorine contained 
in the drainage-water becomes an excellent indication of the 
extent to which the soluble constituents of the manure have been 
washed out of the surface soil; it enables us to judge of the rela- 
tive concentration of the water issuing from different pipes ; it 
also indicates in certain cases whether a mixture of the drainage- 
waters has taken place, A good instance of the important lessons 
which may be learnt from a series of chlorine determinations 
has been already afforded when considering the alteration in 
composition of drainage-waters in different stages of their 
running (Table XLIV.). Facts ascertained with regard to 
chlorides will be equally true of the other soluble diff"usible 
salts present in the soil. 
In Tables XLVI. and XLVH. will be found a summary of the 
amount of chlorine, and of nitrogen, in the form of nitric acid, 
occurring in all the principal runnings of the drain-pipes during 
three years, November 1878 to October 1881. These years 
include long periods of exceptionally high rainfall, in which the 
drains ran with unwonted frequency ; they furnish, in all, instances 
of 49 runnings in which nearly every pipe participated, and thus 
afford material for trustworthy averages, showing the relative 
character of the discharge from each plot. Excepting during 
the first 3^ months, the drainage system of the field was also 
for the whole of this period provided with the later improve- 
ments already mentioned (page 4). 
The form of the Tables is arranged so as to aid the study of the 
