largely thereby. The
process by which beta-naphthylamine is prepared from beta-naphthol,
already referred to, viz. by heating with ammonia under pressure, has been
extended to the sulpho-acids of beta-naphthol, and by this means new
beta-naphthylamine sulpho-acids have been prepared, and figure largely in
the production of these secondary azo-colours. The latter, as previously
stated, possess the most valuable property of dyeing cotton fibre
directly, and by their means the art of cotton dyeing has been greatly
simplified. The shades given by these colours vary from yellow through
orange to bright scarlet, violet, or purple.

In addition to benzidine and tolidine, other diazotisable amido-compounds
have of late years been pressed into the service of the
colour-manufacturer. The derivative of stilbene, already mentioned as
being prepared from a sulpho-acid of one of the nitrotoluenes, forms
tetrazo-salts, which can be combined with similar or dissimilar phenols,
amines, or sulpho-acids, as in the case of benzidine and tolidine.
Various shades of red and purple are thus obtained from the diazotised
compound, when the latter is combined with the naphthylamines, naphthols,
or their sulpho-acids. These, again, are all cotton dyes. The
nitro-derivatives of the ethers of phenol and cresol, when reduced in the
same way that nitrobenzene and nitrotoluene are reduced to azobenzene and
azotoluene, also furnish azo-compounds which, on further reduction, give
bases analogous to benzidine and tolidine. Secondary azo-colours derived
from these bases and the usual naphthalene derivatives are also
manufactured. It is among the secondary azo-dyes that we meet with the
first direct dyeing blacks, the importance of which will be realized when
it is remembered that the ordinary aniline-black is not adapted for wool
dyeing. The azo-blacks are obtained by combining diazotised sulpho-acids
of amidoazo-compounds of the benzene or naphthalene series with naphthol
sulpho-acids or other naphthalene derivatives.

One other series of azo-compounds must be briefly referred to. It has long
been known that aniline and toluidine when heated with sulphur evolve
sulphuretted hydrogen and give rise to thio-bases, that is, aniline or
toluidine in which the hydrogen is partly replaced by sulphur. One of the
toluidines treated in this way is transformed into a thiotoluidine which,
when diazotised and combined with one of the disulpho-acids of
beta-naphthol,