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.
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