nnic
acid and 1 lb. tartar emetic.

(6) DYEING ON METALLIC MORDANTS.

There are a number of dye-stuffs or colouring matters like alizarine,
logwood, fustic, barwood, cutch, resorcine green, etc., which have no
affinity for the cotton fibre, and of themselves will not dye it. They
have the property of combining with metallic oxides such as those of
iron, chromium, aluminium, tin, lead, calcium, etc., to form coloured
bodies which are more or less insoluble in water. These coloured bodies
are called "colour lakes," and the metallic compounds used in connection
with their production "mordants," hence often the dye-stuffs applied by
this method are termed "mordant dyes". In the case of the natural
dye-stuffs--logwood, fustic, Persian berries, Brazil wood, camwood,
cochineal, quercitron, cutch, etc.--which belong to this group of
"mordant dyes," the whole of the material does not enter into the
operation, but only a certain constituent contained therein, which is
commonly soluble in boiling water, and extracted out by boiling. This
constituent is called the "colouring principle" of the dye-stuff or
wood, and naturally varies with each. It is not intended here to deal
in detail with these colouring principles. The methods of applying and
the colours which can be got from these dyes varies very much. Roughly,
the modes of application fall under three heads: (1) the particular
metallic mordant is first fixed on the fibre by any suitable method, and
then the fibre is dyed; (2) the dye-stuff is first applied to the fibre,
and then the colour is fixed and developed by treatment with the
mordant; and (3) the dye-stuff and the mordant are applied at the same
time. This last method is not much used. In the following sections many
examples of these methods will be given.

The dyes fixed with metallic mordant vary in their composition and
properties. There is first the group of eosine dyes, which are acid
derivatives of a colour-base, and, in virtue of being so, will combine
with the metallic oxides. The colour of these colour lakes is quite
independent of what oxide is used, depending only on that of the
particular eosine dye employed. Then there are some members of the azo
dyes, particularly the croceine scarlets, which can also be dyed on the
cotton by the aid of tin, lead or alum mordants. Here, again, the
mordant has no influence on the colour, but only fixes it on the cotton.

The most important class of dye-stuffs which ar