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
Hæmatein, 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 OF WOOL AND FUR; AND OPTICAL PROPERTIES OF COLOURS