we call these compounds naphthols,
and one is distinguished as alpha- the other as beta-naphthol, both of
them having the formula C_{10}H_{7}OH. But now with respect to the
colours. If we treat phenol with nitric acid under proper conditions, we
get a yellow dye called picric acid, which is trinitro-phenol
C_{6}H_{2}(NO_{2})_{3}OH; you see this is no aniline dye; it is not a
basic colour, for it would saturate, _i.e._ destroy the basicity of
bases. Again, by oxidising phenol with oxalic acid and vitriol, we get a
colour dyeing silk orange, namely, Aurin, HO.C[C_{6}H_{4}(OH)]_{3}. This
is also an acid or phenolic dye, as a glance at its formula will show
you. Its compound atom bristles, so to say, with phenol-residues, as
some of the aniline dyes do with aniline residue-groups.

We come now to a peculiar but immensely important group of colours known
as the azo-dyes, and these can be basic or acid, or of mixed kind. Just
suppose two ammonia groups, NH_{3} and NH_{3}. If we rob those nitrogen
atoms of their hydrogen atoms, we should leave two unsatisfied nitrogen
atoms, atoms with an exceedingly keen appetite represented in terms of
hydrogen atoms as N*** and N***. We might suppose a group, though of two N
atoms partially satisfied by partial union with each other, thus--N:N--.
Now this group forms the nucleus of the azo-colours, and if we satisfy a
nitrogen at one side with an aniline, and at the other with a phenol, or
at both ends with anilines, and so on, we get azo-dyes produced. The
number of coal-tar colours is thus very great, and the variety also.

_Adjective Colours._--As regards the artificial coal-tar adjective
dyestuffs, the principal are Alizarin and Purpurin. These are now almost
entirely prepared from coal-tar anthracene, and madder and garancine are
almost things of the past. Vegetable adjective colours are Brazil wood,
containing the dye-generating principle Brasilin, logwood, containing
Haematein, and santal-wood, camwood, and barwood, containing Santalin.
Animal adjective colours are cochineal and lac dye. Then of wood colours
we have further: quercitron, Persian berries, fustic and the tannins or
tannic acids, comprising extracts, barks, fruits, and gallnuts, with
also leaves and twigs, as with sumac. All these colours dye only with
mordants, mostly forming with certain metallic oxides or basic salts,
brightly-coloured compounds on the tissues to which they are applied.

LECTURE XI

DYEING O