ot one of this class.

_Adjective Dyestuffs._--Some of these substances are definitely coloured
bodies, but in some of them the colour is of no consequence or value,
and is quite different and distinct from the colour eventually formed on
the fibre, which colour only appears in conjunction with a special
mordant; but, again, some of them are not coloured, and would not colour
the fibre directly at all, only in conjunction with some mordant. All
the polygenetic colours are, of course, comprised in this class, for
example Alizarin and logwood (Haematein), whilst such monogenetic colours
as annatto and turmeric are substantive, for they will fix themselves
without a mordant on cotton and wool. The adjective colours can be
conveniently subdivided into--(_a_) those existing in nature, as logwood
(Haematein) and Cochineal; (_b_) those artificially formed from coal-tar
products, as Alizarin (madder), Gallein, etc.

_Mineral and Pigment Dyestuffs._--These colours are insoluble in water
and alcohol. They are either fixed on the fibre by mechanical means or
by precipitation. For example, you use blacklead or plumbago to colour
or darken your hats, and you work on this pigment colour by mechanical
means. I will show you by experiment how to fix a coloured insoluble
pigment in the fibre. I take a solution of acetate of lead (sugar of
lead), and to it I add some solution of bichrome (potassium bichromate).
Acetate of lead (soluble in water) with bichromate of potash (also
soluble in water) yields, on mixing the two, acetate of potash (soluble
in water), and chromate of lead, or chrome yellow (insoluble in water),
and which is consequently precipitated or deposited. Now suppose I boil
some of that chrome-yellow precipitate with lime-water, I convert that
chrome yellow into chrome orange. This, you see, takes place without any
reference to textile fibres. I will now work a piece of cotton in a lead
solution, so that the little tubes of the cotton fibre shall be filled
with it just as the larger glass tube or vessel was filled in the first
experiment. I next squeeze and wash the piece, so as to remove
extraneous solution of lead, just as if I had filled my glass tube by
roughly dipping it bodily into the lead solution, and then washed and
cleansed the outside of that tube. Then I place the fabric in a warm
solution of bichromate of potash (bichrome), when it becomes dyed a
chrome yellow, for just as chromate of lead is precipitated in