covered as sulphate of ammonia. It
has been calculated that only from a fifth to a tenth is actually
recovered, and many processes have been patented with a view to
increasing the yield of ammonia in gas manufacture. The total production
of ammonia from gas-works may be placed at little over 100,000 tons per
annum for Great Britain. Mr L. Mond, F.R.S., recently drew attention to
the possibility of largely increasing our supply of sulphate of ammonia
from coal. As indicating what an enormous source of sulphate of ammonia
we have in coal, Mr Mond calculated that its annual consumption in this
country (estimated at 150,000,000 tons) would yield as much as 5,000,000
tons of sulphate of ammonia.

_Other Sources._

While the ammonia produced in the manufacture of gas has long been
collected, it is only of recent years that the other sources of ammonia
have been developed. Next to the gas-works, the shale-works of Scotland
form in this country the chief source of this valuable manure. In these
works the ammonia is obtained in distilling the paraffin shale by a
method somewhat similar to that in use in the gas-works. The amount of
sulphate of ammonia obtained from this source is between 20,000 and
30,000 tons per annum. Recently the ammonia has been recovered from the
blast-furnace gases in iron-works--some 6000 tons being annually
obtained in this way; while from coke and carbonising works the annual
production is about half that amount. The combined annual production
from all these sources may be put down at 140,000 tons, the total
production in Europe being probably little more than 200,000 tons. In
the Appendix further statistics will be found.[214]

_Composition, &c., of Sulphate of Ammonia._

Pure sulphate of ammonia is a whitish crystalline salt, extremely
soluble in water. The commercial article, however, is generally greyish
or brownish in colour, owing to the presence of slight quantities of
impurities. The pure salt should contain 25.75 per cent of ammonia; but
the commercial article is generally sold on a basis of 24.5 per cent. A
useful test of its purity is the fact that when subjected to a red-heat
it should almost entirely volatilise, leaving very little residue. The
chief impurities which it is likely to contain are an excess of
moisture, free acid, or the presence of insoluble matter. Certain
samples contain small quantities of ammonium sulphocyanate, an extremely
poisonous substance for plants. The pr