with
the consumption of coal in London about a century ago, before the
introduction of gas, when, as Bishop Watson tells us in his work already
referred to, the annual consumption was 922,394 tons.

The enormous quantity of tar resulting from our gas manufacture furnishes
the raw material for the production of a multitude of valuable
substances--colouring-matters, medicines, perfumes, flavouring-matters,
burning and lubricating oils, &c. Out of this unsavoury waste material of
the gas-works, the researches of chemists have enabled a great industry to
spring up which is of continually growing importance. It will be the
object of the remaining portion of the present volume to set forth the
achievements of science in this branch of its application. The foundation
of the coal-tar industry was laid in this country--the country where coal
was first distilled on a large scale for the production of gas, and where
the first of the coal-tar colouring-matters was sent forth into commerce.
We are at the present time the largest tar producers in Europe; it has
been stated that we produce more than double the whole quantity of tar
made in the gas-using countries of Europe; but in spite of this, our
manufacture of finished products is by no means in that flourishing
condition which might be expected from our natural resources in the way of
coal, and the facilities which we possess for manufacturing the raw
materials out of it.

But we must now take a glance at some of the other uses to which coal is
put in order to realize more completely the truth of the statement made
some pages back, viz. that this mineral has been the chief source of our
industrial prosperity. Great as is the consumption of coal by the gas
manufacturer, there is an equal or even a greater demand for the
carbonaceous residue left when the coal has been decomposed by destructive
distillation or by partial combustion. This residue is coke--the substance
left in the retorts after the gas manufacture. There is a great demand for
coke for many purposes; it is used in most cases where a cheap smokeless
fuel is required; it is burnt in the furnaces of locomotives and other
engines, and is very largely consumed by the iron smelter in the blast
furnace.

To meet these demands a large quantity of coal is converted into coke by
being burnt in ovens with an insufficient supply of air for complete
combustion, or in suitably constructed close furnaces. The tar and other
prod