a work of this kind. It is important, however, that the
general principles underlying the process of manufacture and the
chemical changes in the phosphate taking place during the process be
clearly understood. In the first place, great importance attaches in the
manufacture of the superphosphate to the fineness of division of the raw
material, and much ingenuity has been spent on apparatus designed for
this purpose. The difficulty of grinding the phosphate varies, of
course, with the nature of the material used--apatite, for example,
being much more difficult to reduce to the necessary fineness than
phosphatic guano. The finer the state of division, the more complete
will be the decomposition of the phosphate by the acid. Mr Warington
recommends that for first-class work the powder should be so fine as to
admit of it passing through a sieve of eighty wires to the inch. After
the phosphate is reduced to powder, it is mixed with acid. This takes
place in the mixer, which is generally in the form of an iron cylinder
furnished in the centre with a revolving shaft, the sulphuric acid used
being the ordinary chamber acid (sp. gr. 1.57). Whatever strength of
acid is used, there must be a certain quantity of water present to form
gypsum. It is to the formation of gypsum in the resulting product that
the dryness of the superphosphate is due. The proportion of sulphuric
acid used depends on the composition of the phosphate; and here it may
be pointed out that the presence of much carbonate of lime is a most
important factor in determining the quantity of acid required. The
reason of this is, that where carbonate and phosphate of lime are
present together, sulphuric acid first acts upon the carbonate, and it
is not till this is wholly decomposed that the phosphate can be acted
upon. Hence mineral phosphates with a large percentage of carbonate of
lime do not constitute such an economical material for the manufacture
of superphosphate as those in which the percentage of carbonate is
small.[225] A certain amount of heat is necessary for the purpose of
enabling a quick decomposition to take place. For this purpose the
sulphuric acid added has been previously heated. In the ordinary
manufacture of superphosphate, however, this is not considered
necessary, as the heat developed by the chemical action between the
phosphate and the acid is sufficiently great. The phosphate, after being
thoroughly mixed with the acid, is discharged into wh