262 The Atmosphere as a source of Nitrogen to Plants. 
as to the sufficiency of the existing methods to determine 
accurately tliis constituent of rain, that they have forborne to 
publish their results at present ; the general conclusion of their 
experiments on this head was, that more nitrogen occurred in the 
form of nitric acid in rain-water than in the form of ammonia, 
which also, as we have seen, was the case in M. Barral's 
experiments. 
Lawes and Gilbert are still, I believe, engaged upon these 
important and interesting experiments.* 
This brief and somewhat imperfect account of the experiments 
made upon this most interesting subject, brings me to consider 
what conclusions, valuable for agricultural theory, and bearing 
upon agricultural practice, may be fairly deduced from them. 
That plants do absorb nitrogen in some form from the air 
seems evident. I have already mentioned natural occurrences, 
which would seem to convince us of this fact. At the same 
time, if this were all the ground for coming to sudh a conclusion, 
we might well hesitate. Recent examinations of the ammonia 
contained in soils, some of them taken at considerable depths, 
and long out of the reach of cultivation, have shown a large 
quantity of this substance to exist in them. Whence was this 
ammonia derived? Not from manure nor from rain, for in one 
case I examined a clay of the plastic-clay formation, dug 20 feet 
from the surface ; it was physically impossible, one would think, 
that either air or water could, in any quantity at least, get access 
to this depth in so close and tenacious a material, yet I found 
more than 1 part of ammonia in 1000 parts of this clay, and I 
ascribed its origin then, as it still appears to me it should be 
ascribed, to the waters of the seas or lakes from which the clay 
was first deposited, and from which, by its absorptive powers for 
ammonia, it had removed this alkali in an insoluble form. The 
existence of ammonia in a soil seems an inherent and inseparable 
result of the presence of clay in the soil, and we may well 
question Avhether all ordinary soils in a state of nature do not 
contain within reach of the roots of plants, especially of large 
trees, sufficient ammonia to account for any accumulation of 
vegetation. It is, however, plain both from M. Ville's and 
M. Boussingault's experiments, that in the absence of all 
ammonia in the soil, plants grown in the open air increase in 
their contents of nitrogen. This point, therefore, is conceded : 
whence then comes this nitrogen ? Ville says — from the nitrogen 
* Mr. Pusey has published, in a late number of the Society's Journal (vol. xiv. 
part ii.), a determination by myself of the ammonia and nitric acid contained in rain 
falling in Berkshire. I did not, as I informed him at the time, place very great re- 
liance on the numbers which 1 obtained, as it is obviously difficult to come at accurate 
results without special arrangements, both for the collection and examination of the 
waters, such as were unlikely to be made for the purpose of a single experiment. 
