part with it in these changes,
which must be of a very complex character, and in the present state of
our knowledge it seems hopeless to attempt any explanation of them.

_Decomposition of Nitric Acid._--Chemists are not entirely at one as to
whether nitric acid is directly absorbed by the plant, or is first
converted into ammonia. But there are certain facts connected with the
chemistry of the soil, to be afterwards referred to, which seem to us to
leave no doubt that it may be directly absorbed; and in that case it
must be decomposed, its oxygen being eliminated, and the nitrogen taking
part with carbon and hydrogen in the formation of the organic compounds.
It must be clearly understood that while such changes as those described
manifestly must take place, the explanations of them which have been
attempted by various chemists are not to be accepted as determinately
established _facts_; they are at present no more than hypothetical views
which have been expressed chiefly with the intention of presenting some
definite idea to the mind, and are unsupported by absolute proof; they
are only inferences drawn from the general bearings of known facts, and
not facts themselves. Although, therefore, they are to be received with
caution, they have advantages in so far as they present the matter to us
in a somewhat more tangible form than the vague general statements which
are all that could otherwise be made.

CHAPTER IV.

THE INORGANIC CONSTITUENTS OF PLANTS.

When treating of the general constituents of plants, it has been already
stated that the older chemists and vegetable physiologists, misled by
the small quantity of ash found in them, entertained the opinion that
mineral matters were purely fortuitous components of vegetables, and
were present merely because they had been dissolved and absorbed along
with the humus, which was then supposed to enter the roots in solution,
and to form the chief food of the plant. This supposition, which could
only be sustained at a time when analysis was imperfect, has been long
since disproved and abandoned, and it has been distinctly shown by
repeated experiment that not only are these inorganic substances
necessary to the plant, but that every one of them, however small its
quantity, must be present if it is to grow luxuriantly and arrive at a
healthy maturity. The experiments of Prince Salm Horstmar, before
alluded to, have established beyond a doubt, that while a seed may