TIte Atmosphere as a source of Nitrogen to Plants. 2G5 
The same is also now demonstrated with regard to nitric acid. 
Cavendish made on this subject an experiment, which has already 
been alluded to. He passed a series of electric sparks through 
air confined over an alkaline liquid, and found that a quantity of 
alkaline nitrate was produced, evidently derived from the union 
of nitrogen and oxygen. His experiment was repeated by Dr. 
Daubeny. 
Since these experiments it has been usual to believe that elec- 
tricity, developed in the higher regions of the air, is continually 
giving rise to nitric acid, and, as it appears that electrical action 
and interchange is always going on, although without the evi- 
dence of its existence which thunder-storms give, there may be 
a constant production of nitric acid from this source. But 
admitting this to be the case, we are surely placed in the dilemma 
of accounting for the absence of any apparent increase in the 
luxuriance of the vegetation of the globe which should follow a 
constant production of nitric acid, unless some equalizing cause 
be in existence, such as the decomposition of ammonia and ex- 
halation of nitrogen observed in Professor Draper's experiments. 
The mutual convertibility of nitric acid and ammonia, as 
shown by Kuhlman to occur under the influence, in the one case, 
of the oxidating influence of the air, and in the other, of the 
de-oxidating influence of vegetable matter in the soil, would 
seem to place these two forms of nitrogen compounds on a level 
as a source of nitrogenous supply, provided the necessary con- 
ditions exist for these changes. And the experience of our agri- 
culturists as to the value of nitrate of soda very sufficiently 
attests the truth of M. Kuhlman's experiments and conclusions. 
We have seen then that in the form of ammonia or nitric 
acid the soil receives annually a very large dose of nitrogen 
in a state to be made use of by plants. That the data yet ob- 
tained are not very precise ought not to surprise us, considering 
the difficulty of the subject. I think, too, that one point has 
been overlooked in all these inquiries : one experimenter devotes 
his attention to the ammonia in rain, another to that in air — both 
independently, at different times, and without concert. But in 
the meanwhile a cause, and, as I believe, a most active cause of 
abstraction of ammonia from the calculations of each of them, is 
at work ; — I mean the absorption of ammonia and nitric acid 
from the air by the soil. 
Between each shower of rain this cause is continually — to an 
unknown, but perhaps a large extent — robbing the air of these 
compounds ; so that the rain when collected really represents 
that which this agency has not removed. On the other hand, the 
quantity at any time present in the air must merely have relation 
to the distance of time at which it was last swept from it by rain, 
