us gases, we need scarcely more than
mention here. But the gas-tar and the ammoniacal liquor are two important
products which demand something more than our casual attention. At one
time regarded by gas engineers as unfortunately necessary nuisances in
the manufacture of gas, they have both become so valuable on account of
materials which can be obtained from them, that they enable gas itself to
be sold now at less than half its original price. The waste of former
generations is being utilised in this, and an instance is recorded in
which tar, which was known to have been lying useless at the bottom of a
canal for years, has been purchased by a gas engineer for distilling
purposes. It has been estimated that about 590,000 tons of coal-tar are
distilled annually.

Tar in its primitive condition has been used, as every one is aware, for
painting or tarring a variety of objects, such as barges and palings, in
fact, as a kind of protection to the object covered from the ravages of
insects or worms, or to prevent corrosion when applied to metal piers.
But it is worthy of a better purpose, and is capable of yielding far more
useful and interesting substances than even the most imaginative
individual could have dreamed of fifty years ago.

In the process of distillation, the tar, after standing in tanks for some
time, in order that any ammoniacal liquor which may be present may rise
to the surface and be drawn off, is pumped into large stills, where a
moderate amount of heat is applied to it. The result is that some of the
more volatile products pass over and are collected in a receiver. These
first products are known as _first light oils_, or _crude coal-naphtha_,
and to this naphtha all the numerous natural naphthas which have been
discovered in various portions of the world, and to which have been
applied numerous local names, bear a very close resemblance. Such an one,
for instance, was that small but famous spring at Biddings, in
Derbyshire, from which the late Mr Young--Paraffin Young--obtained his
well-known paraffin oil, which gave the initial impetus to what has since
developed into a trade of immense proportions in every quarter of the
globe.

After a time the crude coal-naphtha ceases to flow over, and the heat is
increased. The result is that a fresh series of products, known as
_medium oils_, passes over, and these oils are again collected and kept
separate from the previous series. These in turn cease to flow,