o on increasing. Nor must it be forgotten that the
quantity of tar produced in gas-works is now greater than is actually
required by the colour-manufacturer, and much of this by-product is burnt
as fuel, so that if the manufacture of gas were to suffer to any
considerable extent there would still be tar enough to meet our
requirements at the present rate of consumption of the tar-products. Then
again, the value of the tar, coke, and ammoniacal liquor is of such a
proportion as compared with the cost of the raw material, coal, that there
is a good margin for lowering the price of gas when the competition
between the latter and electricity actually comes about. It will not then
be only a struggle between the two illuminants, but it will be a question
of electricity _versus_ gas, _plus_ tar and ammonia.

While the electrician is pushing forward with rapid strides, the chemist
is also moving onwards, and every year witnesses the discovery of new tar
products, or the utilization of constituents which were formerly of little
or no value. Thus if the cost of generating and distributing electricity
is being lowered, on the other hand the value of coal tar is likely to go
on advancing, and it would be rash to predict which will come out
triumphant in the end. But even if electricity were to gain the day it
would be worth while to distil coal at the pit's mouth for the sake of the
by-products, and there is, moreover, the tar from the coke ovens to fall
back upon--a source which even before the use of coal-gas the wise Bishop
of Llandaff advised us not to neglect.

CHAPTER II.

The nature of the products obtained by the destructive distillation of
coal varies according to the temperature of distillation, and the age or
degree of carbonization of the coal. The watery liquor obtained by the dry
distillation of wood is acid, and contains among other things acetic acid,
which is sometimes prepared in this way, and from its origin is
occasionally spoken of as "wood vinegar." The older the wood, the more
complete its degree of conversion into coal, and the smaller the quantity
of oxygen it contains, the more alkaline does the watery liquid become.
Thus the gas-liquor is distinctly alkaline, and contains a considerable
quantity of ammonia, besides other volatile bases. The uses of ammonia are
manifold, and nearly our whole supply of this valuable substance is now
derived from gas-liquor. The presence of ammonia in this liquor is