effected under a pressure somewhat less than that
of the atmosphere, the products of distillation being pumped out of the
retorts by means of a kind of air-pump, called an exhauster, which is
interpolated between the hydraulic main and the condenser, or at some
other part of the purifying system. The coke left in the retort is used as
fuel for burning under the retorts or for other purposes. The oxide of
iron used in the purifiers can be used over and over again for a certain
number of times by exposing it to the air, and when it is finally
exhausted, the sulphur can be burnt out of it and used for making that
most important of all chemical products, sulphuric acid. Thus the small
quantity of sulphur present in the original coal (probably in the form of
iron pyrites) is rendered available for the manufacture of a useful
product.

The necessarily brief description of this important industry will suffice
for the general reader. Those who desire further information on points of
detail will refer to special works. We are here rather concerned with the
subsequent fate of the different products, four of which have to be dealt
with, viz. the gas, watery liquor, tar, and coke. The first and last of
these having already been accounted for--the one as an illuminating agent
and the other as fuel--may now be dismissed.

No story of applied science is complete unless we can form some idea of
the quantities of material used, and the amount of the products obtained.
From one ton of Newcastle coal we get about 10,000 cubic feet of gas, 110
to 120 lbs. of tar, 20 to 25 gallons of watery liquor, and about 1500 lbs.
of coke. Different coals of course give different quantities, and the
latter vary also according to the heat of distillation; but the above
estimate will furnish a good basis for forming our ideas with some
approach to precision. It has been estimated also that we are now
distilling coal at the rate of about ten million tons per annum, so that
there is annually produced 100,000 million cubic feet of gas, and about
500,000 tons of tar, besides proportionate quantities of the other
products. The great metropolitan companies alone are consuming nearly
three million tons annually for the production of gas, a consumption
corresponding to about 6000 cubic feet per head of the population. This of
course takes no account of the coal used for other manufactures or for
domestic purposes, but it is interesting to compare these estimates