264 The Atmosphere as a source of Nitrogen to Plants. 
water of fogs and clews than in rain, and in the first showers than 
in those that fall subsequently — that as a consequence of the 
existence of ammonia in rain-water, it is found also in the water 
of streams and rivers, and further on still, in the great ocean ; but 
that the quantity in these cases is much smaller than in rain, 
owing to causes already alluded to. 
Now whence come these ingredients of the air ? In the first 
place ammonia is given off in the decomposition of all animal 
matters, and of vegetable matters containing nitrogen, and in animal 
perspiration. Mr. Lawes has also found a great loss of ammonia 
from the soil in the growth of wheat and other cereal crops : that 
is to say, he finds tliat for 1 pound of nitrogen fixed in the 
composition of the plant, about 4 pounds are taken from the re- 
sources of the soil. Now as this does not occur with other plants, 
at all events to the same extent, it is obvious that this loss is not 
from evaporation from the soil direct, but through the agency of 
the plant. Professor Draper's experiments would lead us to 
believe that ammonia is under some circumstances decomposed, 
and its nitrogen exhaled by plants. It will be remembered that 
on the other hand from certain peculiarities of silicate of 
ammonia, I have suggested that the loss of the alkali observed 
by Mr. Lawes may be due to the ammonia acting as a carrier to 
silica, and that it is hence only seen in plants having a silicious 
character. 
Boussingault appears to think that ammonia in some way 
escapes from the soil, and in this way accounts for snow which 
has lain on the ground being rich in this alkali. It is generally 
thought by farmers, that in very hot, dry weather, guano and 
similar manures are, even when to a certain extent mixed with the 
soil, liable to loss ; but this is only in the absence of rain to bring 
them well into contact with the soil, and on the other hand it is 
very generally believed that in the coldest weather manures may 
be laid on the surface without loss. Whereas M. Boussingault's 
experiment leading to the inference of the evaporation of ammonia 
from the soil was of course made in cold weather. The con- 
clusion that under circumstances of perfect mixture with the soil 
ammoniacal compounds are evaporated from it, seems to me 
utterly opposed to the fact which I have demonstrated, that when 
air charged with the ammoniacal vapours is passed over a soil 
contained in a tube, the ammonia is entirely removed. 
Without, however, being over anxious as to how ammonia* 
comes to be present in the air, or what was its first origin, we 
may be satisfied that air does always contain it in notable quantity. 
* Mulder believes that ammonia may be formed from the nitrogen of the air 
uniting with the hydrogen of decomposing vegetable matters in the soil. 
