his is similarly washed and purified by distillation. The large
quantity of naphthalene existing in tar has already been referred to, but
although it is such an important constituent, it was only late in the
history of the colour industry that it found any extensive application. In
early times it was regarded as a nuisance, and was burnt as fuel, or for
the production of a dense soot, which was condensed to form lampblack. It
will be remembered that the first of the coal-tar colours made required
only the light oils. There are at present only a few direct uses for
naphthalene, but one of its applications is sufficiently important to be
mentioned.

The hydrocarbon is a white crystalline solid melting at 80 deg. C., and
boiling at 217 deg. C. Although it has a high boiling-point, it passes readily
into vapour at lower temperatures, and the vapour on condensation forms
beautiful silvery crystalline scales. This product is "sublimed
naphthalene." The vapour of naphthalene burns with a highly luminous
flame, and if mixed with coal-gas, it considerably increases the
luminosity of the flame. Advantage is taken-of this in the so-called
"albo-carbon light," which is the flame of burning coal-gas saturated with
naphthalene vapour. The burner is constructed so that the gas passes
through a reservoir filled with melted naphthalene kept hot by the flame
itself (Fig. 9).

[Illustration: FIG. 9.--ALBO-CARBON BURNER.]

To appreciate properly the value of those discoveries which have enabled
manufacturers to utilize this hydrocarbon, it is only necessary to recall
to mind the actual quantity produced in this country. Supposing that ten
million tons of coal are used annually for gas-making, and that the
500,000 tons of tar resulting therefrom contain only eight per cent. of
naphthalene, there would be available about 40,000 tons of this
hydrocarbon annually. Great as have been the recent advancements in the
utilization of naphthalene derivatives, there is still a larger quantity
of this hydrocarbon produced than is necessary to supply the wants of the
colour-manufacturer. From this last statement it will be inferred that
naphthalene is now a source of colouring-matters. Let us consider how this
has been brought about.

The phenols of naphthalene are called naphthols--they bear the same
relationship to naphthalene that carbolic acid bears to benzene. Owing to
the structure of the naphthalene molecule there are two isomeric
naphtho