c matter, by which it is brought into
a condition capable of undergoing a more rapid change in the soil. The
rapidity of action of bones is still more promoted by solution in
sulphuric acid, by which they are converted into the form of dissolved
bones or superphosphate. At the present moment, however, very little of
the superphosphates sold in the market are made exclusively from bones
in their natural state, by far the larger portion being manufactured
from mineral phosphates, or from bones after destruction of their
organic matter, sometimes with the addition of small quantities of
unburnt bones, but more frequently of sulphate of ammonia, to yield the
requisite quantity of ammonia. These substances may therefore be best
considered under the head of mineral manures.

CHAPTER XI.

COMPOSITION AND PROPERTIES OF MINERAL MANURES.

Mineral manure is a term which is now used with great laxity. In its
strict sense, it means manures which contain only, and owe their
exclusive value to the presence of, those substances which go to make up
the inorganic part or ash of plants. It has, however, been usually taken
to include all saline matters, and especially the compounds of ammonia
and nitric acid, which are indebted for their manurial effects to the
nitrogen they contain; and thus is so far incorrect. It would, however,
be manifestly impossible to arrange these compounds with any degree of
accuracy among either animal or vegetable manures, and hence the
necessity of including them amongst those which are strictly mineral.
The most important practical distinction between them and the substances
discussed in the two preceding chapters is, that the latter generally
contain the whole or the greater part of the constituents of plants.
Even bones yield a certain quantity of alkalies, magnesia, sulphuric
acid, and chlorine, and may in some sense be considered as a general
manure. But those to which the term mineral manure is applied for the
most part contain only one or two of the essential elements of plants,
and hence cannot be applied as substitutes for the substances already
discussed, although they are frequently most important additions to
them.

_Sulphate and Muriate of Ammonia._--These and other salts of ammonia
have been tried experimentally as manures, and it has been ascertained
that they may all be used with equal success; but as the sulphate is by
much cheaper, it is that which probably will always be employed