en
made to contrive a general system suited to all manures, and which,
though not absolutely correct, is a sufficient approximation for all
practical purposes, and a tolerably accurate guide to the determination
of their relative values.

The constituents of a manure which are of actual value are ammonia,
insoluble phosphates, biphosphate of lime (soluble phosphates), sulphate
of lime, nitric acid (as nitrate of soda), potash, soda, and organic
matter. These substances differ greatly in value. Ammonia and
phosphates, soluble and insoluble, are costly; and by far the larger
part of the value of all guanos, and the common manufactured manures,
depends on them. Nitric acid and potash are also very valuable
substances, but as they are rarely found in manufactured manures, and
never in sufficient quantity to exert any material influence in their
price, it is not usual to take them into consideration except in
particular cases. The alkali which commonly exists in artificial manures
is soda, and when alkaline salts appear in any analysis, they must be
assumed to consist almost entirely of that substance generally in the
form of common salt, and be valued accordingly. Sulphate of lime and
organic matter though abundant constituents of most manures, add but
little to their value, and it is a moot point whether they ought to be
taken into consideration, although most persons allow a small value for
them. Carbonate of lime, sand, or siliceous matter, and water, of
course, are altogether worthless.

In order to obtain the value of a manure containing several of these
substances, it is necessary to ascertain the average commercial price of
each individually. This is easily done when they are met with in
commerce separately, or at least mixed only with worthless substances,
but some of them are only found in complex mixtures, and in these cases
it is necessary to arrive at a result by an indirect process, according
to methods which will be immediately explained. The question to be
solved is the price actually paid for a ton of each substance in a pure
state, and we shall proceed to consider them in succession.

_Insoluble Phosphates._--These are purchased alone, chiefly in the form
of coprolites and bone-ash, or the spent animal charcoal of the sugar
refiners. Ground coprolites, containing about 58 per cent of phosphates,
sell at L2: 12s. per ton, which is at the rate of L4: 8s. for pure
phosphates. Bone-ash varies considerably in