at is technically
known as the pit, a chamber built of brick or concrete. The mixture,
which is in a fluid state when it enters the pit, very soon hardens, and
is dug out in a day or two. It is next reduced to powder in a
disintegrator, and is then ready for use as a manure.

_Nature of the Reaction taking place._

In order to clearly understand the nature of the reaction which takes
place when sulphuric acid is added to a phosphatic material, it may be
well to say a word or two on the composition of the different compounds
of lime and phosphoric acid.

_Phosphates of Lime._

In the various phosphatic manures used in agriculture there are four
different kinds of phosphates. In the commonest form, popularly called
bone-phosphate, which is the form in which lime and phosphoric acid are
combined in bones, guano, and the ordinary mineral phosphates, the lime
and phosphoric acid are combined in the form of what is known as
tribasic phosphate of lime, or tricalcic phosphate--that is to say, for
every equivalent of phosphoric acid there are three equivalents of lime.
This may be represented as follows:--

Lime }
Lime > Phosphoric acid.
Lime }

Or we may also say that for every 142 parts by weight of phosphoric acid
there are 168 parts by weight of lime in this form of phosphate. This is
the least soluble form of phosphoric acid,[226] and is the form
generally referred to in commercial analyses as insoluble phosphate.
When this phosphate is acted upon with sulphuric acid, a soluble
phosphate is formed, as Liebig first showed, to which the name
superphosphate has been given, and which is also known as monobasic
phosphate of lime, or monocalcic phosphate. This compound may be
represented as containing, instead of three equivalents of lime, only
one, the other two equivalents being replaced by water. This compound
may be represented as follows:--

Lime }
Water > Phosphoric acid.
Water }

In it, for every 142 parts of phosphoric acid, there are only 56 parts
of lime. It is soluble in water, and gives to the commercial article
known as superphosphate of lime its value. Intermediate in composition
between these two phosphates there is another known as precipitated
phosphate of lime, or dicalcic phosphate (the same as reverted
phosphate), which contains two equivalents of lime and one equivalent of
water as follows:--

Lime }
Lime > Phosphoric acid.
Water }

This compound contains, for every 142 parts of p