covery of the parent hydrocarbon of this
colouring-matter which had been in use for so many ages--a
colouring-matter capable of furnishing both in dyeing and printing many
distinct shades, all possessed of great fastness--was obviously a step
towards the realization of an industrial triumph, viz. the chemical
synthesis of alizarin. Within a year of their original observation, this
had been accomplished by Graebe and Liebermann, and almost simultaneously
by W. H. Perkin in this country. From that time the anthracene, which had
previously been burnt or used as lubricating grease, rose in value to an
extraordinary extent. In two years a material which could have been bought
for a few shillings the ton, rose at the touch of chemical magic to more
than two hundred times its former value.

Anthracene is a white crystalline hydrocarbon, having a bluish
fluorescence, melting at 213 deg. C. and boiling above 360 deg. C. It was
discovered in coal-tar by Dumas and Laurent in 1832, and its composition
was determined by Fritzsche in 1857. It separates in the form of crystals
from the anthracene oil on cooling, and is removed by filtration. The
adhering oil is got rid of by submitting the crystals to great pressure in
hydraulic presses. Further purification is effected by powdering the crude
anthracene cake and washing with solvent naphtha, _i.e._ the mixture of
the higher homologues of benzene left after the rectification of the light
oil. Another coal-tar product, viz. the pyridine base referred to in the
last chapter, has been recently employed for washing anthracene with great
success. It is used either by itself or mixed with the solvent naphtha.
The anthracene by washing with these solvents is freed from more soluble
impurities, and may then contain from 30 to 80 per cent. of the pure
hydrocarbon. The washing liquid, which is recovered by distillation,
contains, among other impurities dissolved out of the crude anthracene, a
hydrocarbon isomeric with the latter, and known as phenanthrene, for which
there is at present but little use, but which may one day be turned to
good account. The actual amount of anthracene contained in coal-tar
corresponds to about 1/2 lb. per ton of coal distilled, _i.e._ from 1/4 to
1/2 per cent. by weight of the tar. Owing to the great value of alizarin
and the large quantity of this colouring-matter annually consumed,
anthracene is now by far the most important of the coal-tar hydrocarbons.

Alizarin