ined in the plants must, in this instance, have
been obtained entirely from the nitrate of potash, for the quantity
contained in it and in the seeds is exactly equal to that in the plants
and the soil, the difference of 0.03 grains being so small that it may
be safely attributed to the errors inseparable from such experiments.
For the sake of comparison, an exactly similar experiment was made on
two seeds grown without nitrate of potash, and in this instance, after
an equally long period of growth, the largest plant had only attained a
height of 7.5 inches, and had three small pale and imperfectly developed
leaves. They contained only 0.033 grains of nitrogen, while the seeds
contained 0.032--indicating that, under these circumstances, there was
no increase in the quantity of that element.

But, independently of these experimental results, it may be inferred
from general considerations, that nitric acid must be one of the sources
from which plants derive their nitrogen. It has been already stated,
that the humus contained in the soil consists of the remains of decayed
plants, and there is every reason to suppose that the primeval soil
contained no organic matters, and that the first generation of plants
must have derived the whole of their nitrogen from, the atmosphere. If,
therefore, it be assumed that ammonia is the only source of the
nitrogen of plants, it would follow, that as that substance cannot be
produced by the direct union of its elements, the quantity of ammonia in
the air could only remain undiminished in the event of the whole of the
nitrogen of decaying plants returning into that form. But this is
certainly not the case, for every time a vegetable substance is burned,
part of its nitrogen is liberated in the free state, and in certain
conditions of putrefaction, nitric acid is produced. Now, if ammonia be
the only form in which nitrogen is absorbed, there must be a gradual
diminution of the quantity contained in the air; and further, there must
either be some continuous source of supply by which its quantity is
maintained, or there must be some other substance capable of affording
nitrogen in a form fitted for the maintenance of plant life. As regards
the first alternative, it must be stated that we know of no source other
than the decomposition of plants from which ammonia can be derived, and
we are therefore compelled to adopt the second alternative, and to admit
that there must be some other source of n