ure containing a larger proportion of
toluene, then comes a purer toluene mixed with a small percentage of
xylene. The boiling-points of the three hydrocarbons are 81 deg. C., 111 deg. C.,
and 140 deg. C. respectively; but owing to the nature of fractional
distillation, a compound of a certain boiling-point always brings over
with it a certain quantity of the compound of higher boiling-point, and
that is why the rectifying column effects only a partial separation.

Of the hydrocarbons thus separated, benzene and toluene are by far the
most important; there is but a limited use for the xylenes at present, and
these and the hydrocarbons of higher boiling-point belonging to the same
series which distil over between 140 deg. and 150 deg. constitute what is known as
"solvent naphtha," because it is used for dissolving india-rubber for
waterproofing purposes. The hydrocarbons of still higher boiling-point
which remain in the still are used as burning naphtha for lamps. If
benzene of a higher degree of purity is required--as it is for the
manufacture of certain colouring-matters--the fraction containing this
hydrocarbon can be again distilled through the rectifying column, and a
large proportion of the toluene thus separated from it. Finally, pure
benzene can be obtained by submitting the rectified hydrocarbon to a
process of refrigeration in a mixture of ice and salt, when the benzene
solidifies to a white crystalline solid, while the toluene does not
solidify, and can be drained away from the benzene crystals which liquefy
at about 5 deg. C.

The account rendered by the technologist with respect to the light oils of
the tar is thus a pretty good one. Already we see that benzene, toluene,
solvent naphtha, and burning naphtha are separated from them. Even the
alkaline and acid washings may be made to surrender their contained
products, for the first of these contains a certain quantity of carbolic
acid, and the acid contains a strongly smelling base called pyridine, for
which there is at present no great demand, but which may one day become of
importance. The actual quantity of benzene in tar is a little over one per
cent. by weight, and of toluene there is somewhat less. The naphthas are
present to the extent of about 35 per cent.

Now let us consider some of the transformations which benzene and toluene
undergo in the hands of the manufacturing chemist. The production of
aniline from benzene by acting upon this hydrocarbon