he combined tannic acid to unite
also with the basic aniline colours, forming very fast or insoluble
colour lakes. This principle is extensively used in practice to fix
basic aniline colours, especially on cotton. We should first soak the
piece of cotton in a solution of tannic acid, and then pass it into a
solution, say, of tartar emetic, when the tannic acid will be firmly
fixed, as tannate of antimony, on the cotton. We then dip the mordanted
piece of cotton into the colour bath, containing, for instance, Magenta,
and it is dyed a fine red, composed of a tannate of antimony and
Magenta. You now see, no doubt, the necessity of sharply discriminating
between two classes of colouring matters, which we may term _colour
acids_ and _colour bases_ respectively. There are but few acids that act
like tannic acid in fixing basic aniline dyestuffs, but oleic acid and
other fatty acids are of the number. A curious question might now be
asked, namely: "Could the acid colour Alizarin, if fixed on cotton
cloth, combine with a basic aniline colour, _e.g._ Aniline Violet, and
act as a mordant for it, thus fixing it?" The answer is, "Certainly";
and thus an Alizarin Purple would be produced, whilst if Magenta were
used in place of Aniline Violet, an Alizarin Red of a crimson tone would
result.

_Chrome Mordanting of Wool and Fur._--In studying this subject I would
recommend a careful perusal of the chapter on "Mordants" in J.J.
Hummel's book, entitled _The Dyeing of Textile Fabrics_, and pages 337
to 340 of Bowman's work on _The Wool-Fibre_.

In the treatment of wool or fur with bichrome (potassium bichromate) we
start with an acid salt, a bichromate (K_{2}Cr_{2}O_{7}) and a strong
oxidising agent, and we finish with a basic substance, namely, oxide of
chromium, in the fibres of the wool or fur. If we desire to utilise the
whole of the chromic acid in our mordanting liquor, we must add to it
some sulphuric acid to set free the chromic acid from the potassium with
which it is combined. Bichromate of potash with sulphuric acid gives
sulphate of potash and chromic acid. The question of the proper
exhaustion of bichromate baths is an important economic one. Now we must
remember that this chromic acid (CrO_{3}) oxidises our wool or fur, and
must oxidise it before it can of itself act as a mordant by being
reduced in the process to hydrated chromic oxide, Cr_{2}O_{3} + 3
H_{2}O. [2 CrO_{3} (chromic acid) = Cr_{2}O_{3} (chromic oxide) + O_{