recognised. At first it was thought
that it was impossible to estimate its quantity by chemical analysis.
This difficulty, however, has been overcome, and it is generally
admitted that the ammonium citrate process furnishes an accurate means
of determining its amount. Both on the Continent and in the United
States reverted phosphate is recognised as possessing a monetary value
in excess of that possessed by the ordinary insoluble phosphate. The
result is, that raw phosphates containing iron and alumina to any
appreciable extent are not used in this country, although they do find a
limited application in America and on the Continent.

_Composition of Superphosphates._

Superphosphates as manufactured may be divided, generally speaking, into
three classes--viz., low class, medium, and high class. The ordinary or
medium class contains from 25 to 27 per cent of soluble phosphate; and
here it may be pointed out that by soluble phosphate is meant the
percentage of tribasic phosphate which has been dissolved--not, as might
at first sight be supposed, the percentage of monocalcic phosphate. The
lower-class superphosphates are those containing less than 25 per cent,
generally 23 to 25 per cent, of soluble phosphate; while the high-class
superphosphate may contain from 30 to 45 per cent. For the manufacture
of high-class superphosphate only a certain number of raw phosphates are
available, such as Curacao and Somme phosphates, phosphatic guanos,
bone-char, &c. Certain processes have been patented for the manufacture
of even more concentrated superphosphates, and by them phosphates
containing as much as 40 per cent of soluble phosphoric acid--_i.e._,
equal to 87 per cent of soluble phosphate--have been prepared. To this
class belongs the so-called double superphosphate, manufactured at
Wetzlar in Germany. Such a concentrated form of manure is naturally very
expensive to manufacture, and is hardly to be recommended for home
consumption. Where, however, manures have to be conveyed long distances,
and the freight is consequently very high, such a concentrated article
may be found most economical.

_Action of Superphosphates._

When superphosphate is applied to the soil it is converted into an
insoluble state. In short, the process of reversion is carried on on a
wholesale scale. This is due to the lime, iron, and alumina salts which
the soil contains. In all probability the phosphate is finally converted
into a hydrated ferri