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 the glass tube, so it is now precipitated in the little tubes of the cotton fibre (see Lecture I.).  Let us see if we can now change our chrome yellow to chrome orange, just as we did in the glass vessel by boiling in lime-water.  I place the yellow fabric in boiling lime-water, when it is coloured or dyed orange.  In each little tubular cotton fibre the same change goes on as went on in the glass vessel, and as the tube or glass vessel looks orange, so does the fabric, because the cotton fibres or tubes are filled with the orange chromium compound.  You see this is quite a different process of pigment colouring from that of rubbing or working a colour mechanically on to the fibre.

Let us now turn to the substantive colours (Group I.), and see if we can further sub-divide this large group for the sake of convenience.  We can divide the group into two—­(a) such colours as exist ready formed in nature, and chiefly occur in plants, of which the following are the most important:  indigo, archil or orchil, safflower, turmeric, and annatto; (b) the very large sub-group of the artificial or coal-tar colours.  We will briefly consider now the dyestuffs mentioned in Group (a).