there may be several that each is capable of yielding. We may consider
them as colour-giving principles, and so we term them _polygenetic_
colours. Polygenetic means capable of generating several or many. In the
various colours and dyes we have all phases, and the monogenetic shades
almost imperceptibly into the polygenetic. The mode of application of
the two classes of colours is, of course, in each case quite essentially
different, for in the case of the monogenetic class the idea is mainly
either to dye at once and directly upon, the unprepared fibre, or having
subjected the fabric to a previous preparation with a metallic or other
solution, to fix directly the one colour on that fabric, on which,
without such preparation, it would be loose. In the case of the
polygenetic class, the idea is necessarily twofold. The dyeing materials
are not colours, only colour generators. Hence in all cases the fabric
must be prepared with the twofold purpose--first, of using a metallic or
other agent, capable of yielding, with the dye material, the desired
colour; and secondly, of yielding it on the fibre in an insoluble and
permanent form. Now, though I have gone so far into this mode of
classification, because it does afford some information and light, yet I
can go no farther without getting into a territory that presupposes a
knowledge and acquaintance with the chemical structure of the colouring
matters as organic substances, which would be, at present, beyond us. I
shall now turn to another mode of classification, which, if not so
far-reaching as the other, is at least an exceedingly useful one. The
two methods may be combined to a considerable extent. By the latter plan
the colours may be conveniently divided into three groups: I.,
substantive colours; II., adjective colours; III., mineral and pigment
colours.

_Substantive Dyestuffs._--The substantive colours fix themselves readily
and directly on animal fibres and substances, but only a few amongst