the light
oil boiling above toluene were of no value in the colour industry. Benzene
and toluene are related to each other in a way which chemists describe by
saying that they are "homologous." This means that they are members of a
regularly graduated series, the successive terms of which differ by the
same number of atoms of carbon and hydrogen. Thus toluene contains one
atom of carbon and two atoms of hydrogen more than benzene. Above toluene
are higher homologues, viz. xylene, cumene, &c., which occur in the light
oil, the former being related to toluene in the same way that toluene is
related to benzene, while cumene again contains one atom of carbon and two
atoms of hydrogen more than xylene. This relationship between the members
of homologous series is expressed in other terms by saying that the weight
of the molecule increases by a constant quantity as we ascend the series.

The homology existing among the hydrocarbons extends to all their
derivatives. Thus phenol is the lower homologue of the cresols. Also we
have the homologous series--

Benzene. Nitrobenzene. Aniline.
Toluene. Nitrotoluene. Toluidine.
Xylene. Nitroxylene. Xylidine.
Cumene. Nitrocumene. Cumidine.

The bases of the third column when diazotised and combined with the
disulpho-acids of beta-naphthol give a graduated series of dyes beginning
with orange and ending with bluish scarlet. Thus it was observed that the
toluidine colour was redder than the aniline colour, and it was a natural
inference that the xylidine colour would be still redder. At the time of
this discovery no azo-colour of a true scarlet shade had been manufactured
successfully. A demand for the higher homologues of benzene was thus
created, and the higher boiling-point fractions of the light oil, which
had been formerly used as solvent naphtha, became of value as sources of
colouring-matters. The isolation of coal-tar xylene (which is a mixture of
three isomeric hydrocarbons) is easily effected by fractional distillation
with a rectifying column, and by nitration and reduction, in the same way
as in the manufacture of aniline, xylidine is placed at the disposal of
the colour-maker.

Xylidine scarlet, although at the time of its introduction the only true
azo-scarlet likely to come into competition with cochineal, was still
somewhat on the orange side. The cumidine dye would obviously be nearer
the desired shade. To meet this