hosphoric acid, 112
parts of lime; and in solubility occupies an intermediate position.
Lastly, there is a fourth compound of lime and phosphoric acid, which
only occurs in one phosphatic manure--viz., phosphatic slag, in which
indeed it was first discovered--which consists of four equivalents of
lime to one of phosphoric acid, to which the name tetrabasic phosphate
of lime or tetracalcic phosphate has been given. Its composition may be
illustrated as follows:--

Lime }
Lime > Phosphoric acid.
Lime >
Lime }

Or, for every 142 parts of phosphoric acid, there are 224 parts of lime.
Contrary to what we might expect, this phosphate is less insoluble than
the ordinary tribasic or bone phosphate. This may be owing to the fact
that, in the tetrabasic phosphate, there is more lime present than that
which the phosphoric acid can retain with strong chemical affinity.[227]
In the manufacture of superphosphate the tribasic phosphate is converted
into the soluble phosphate--the lime, which was formerly in combination
with the phosphoric acid, uniting with the sulphuric acid, and forming
gypsum.[228] It was till recently supposed that soluble phosphate and
gypsum were the only two resulting products of this decomposition. It
has been recently shown, however, by Ruffle and others, that this is
not, strictly speaking, the case, and that probably a large proportion
of free phosphoric acid is formed; in fact, it seems probable that in
the first stage of the reaction, only phosphoric acid is produced, and
that this subsequently acts upon the undecomposed phosphate, with the
production of monocalcic phosphate.[229] The amount of sulphuric acid
which experience has shown it is necessary to add for the successful and
economical manufacture of superphosphate, depends on the composition of
the raw material employed. The larger the percentage of tribasic
phosphate, the larger the quantity of sulphuric acid required for its
decomposition; but sometimes even a poor phosphate consumes a large
amount of sulphuric acid. This is the case where much calcium carbonate
or fluoride is present in the raw phosphate, as both of these compounds
require a quantity of acid for their decomposition, which takes place
before the decomposition of the phosphate. Hence phosphates rich in
carbonate of lime are not well suited as economical materials from which
to manufacture superphosphate.

_Reverted Phosphates._

A change which is apt to take place in