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