pecially on leguminous plants, but they do not
afford an explanation of its mode of action, for which further
inquiries, directed especially to that object, are required.

The application of gypsum to the soil appears to have diminished of late
years, and this is probably due to the large consumption of
superphosphates, and other manufactured manures, which contain it in
abundance. In an ordinary application of these substances, there are
contained from one to two hundredweight of gypsum; and it is not likely
that when they have been extensively used, much benefit will be derived
from a further application of it by itself.

CHAPTER XII.

THE VALUATION OF MANURES.

The determination of the value of a manure is in many respects a
commercial rather than a chemical question, but as it must be founded on
the analysis, and presents some peculiarities dependent on the
complicated nature of the substances to be valued, it has fallen to some
extent into the hands of the chemist. The principle on which the value
of any commercial sample is estimated is very simple. It is only
necessary to know the price of the pure article, and that of the
particular sample to be valued is obtained by making a deduction from
this price proportionate to the per centage of impurities shewn by the
analysis. Thus, for example, if pure sulphate of ammonia sells at L16
per ton, a sample containing 10 per cent of impurities ought to be
purchased for L14: 8s., and so on for any other quantity. This system
which answers perfectly with sulphate of ammonia, nitrate of soda, or
any other substance whose value depends on one individual element, is
inapplicable in the case of complex manures, such as guano and the like,
in which several factors combine to make up the value. In such cases,
manures of very different composition may have the same value, the
deficiency in one particular element being counterbalanced by the excess
of another. Hence it becomes necessary to obtain an estimate of the
value of each factor, from which that not only of one particular
substance, but of every possible mixture may be determined.

When we come to inquire minutely into this question, it appears that the
commercial value of any substance is not estimated solely by
considerations of composition, but is dependent to a great extent on
questions of demand and supply, and applicability to particular
purposes. Thus coprolites containing from 55 to 60 per cent of
phosph