lts = 1.68 phosphate of lime }

These substances are all excellent sources of phosphates, but they are
so hard that the plants cannot extract phosphoric acid from them, and
they are only useful when made soluble by chemical processes.

_Superphosphate; Dissolved Bones._--These names were at first applied to
bones which had been treated with sulphuric acid; but superphosphates
are now rarely made from bones alone, but bone ash and some of the
mineral phosphates just described are employed, either along with them,
or very frequently alone. The manufacture of superphosphates depends on
the existence of two different compounds of phosphoric acid and lime,
one of which contains three times as much lime as the other. That which
contains the larger quantity of lime is found in the bones and all other
natural phosphates, and is quite insoluble in water; but when two-thirds
of its lime are removed, it is converted into the other compound, which
is exceedingly soluble. This change is effected by the use of sulphuric
acid, which combines with two-thirds of the lime of the ordinary
insoluble phosphate of lime, and converts it into the _biphosphate of
lime_, which is soluble. When, therefore, we add to 100 lbs. of common
phosphate of lime the necessary quantity of sulphuric acid, it yields 64
lbs. of biphosphate, containing the whole of the phosphoric acid, which
is the valuable constituent, the diminution in weight being due to the
removal of the valueless lime. Hence it follows, also, that as the lime
so removed is converted into sulphate, there must, for every 100 lbs. of
phosphate of lime converted into biphosphate, be produced 87 lbs. of dry
sulphate of lime, or 110 of the ordinary sulphate called gypsum. This is
the minimum quantity which can be present, but in actual practice it is
liable to be greatly exceeded, more especially where coprolites are
used, owing to the large amount of carbonate of lime they contain, which
is also converted into sulphate by the action of the acid, so that it is
far from uncommon to find the gypsum twice as great as it would be if
materials free from carbonates could be obtained. By employing a
sufficiency of sulphuric acid, the whole quantity of phosphoric acid in
the bones may be thus brought into a soluble state, but in actual
practice it is found preferable to leave part of it in the insoluble
condition; as where it is entirely soluble, its effect is too great
during the early part of the sea