tion which is called a "colour lake," which
consists of at least two constituents; one of these is the dye-stuff or
the colouring matter itself, the other being either the fibre or a
mordant, if such has to be used. The question of the formation of colour
lakes is one connected with the molecular constitution of the colouring
matter, but much yet remains to be done before the proper functions and
mode of action of the various groups or radicles in the dye-stuffs can
be definitely stated. While the constitution of the dye-stuff is of
importance, that of the substance being dyed is also a factor in the
question of the conditions under which it is applied.

In dealing with the first of the above groups of dyes, the direct dyes,
the colourist is somewhat at a loss to explain in what manner the
combination with the cotton fibre is brought about. The affinity of
cellulose for dyes appears to be so small and its chemical activities so
weak, that to assume the existence of a reaction between the dye-stuff
and the fibre, tending to the formation of a colour lake, seems to be
untenable. Then, again, the chemical composition and constitution of the
dyes of this group are so varied that an explanation which would hold
good for one might not do so for another. The relative fastness of the
dyes against washing and soaping precludes the idea of a merely
mechanical absorption of the dye by the fibre; on the other hand the
great difference in the fastness to soaping and light between the same
dyes on cotton and wool would show that there has not been a true
formation of colour lake.

The dyeing of cotton with the second group of dyes is more easily
explained. The cotton fibre has some affinity for the tannic acid used
in preparing it and absorbs it from the mordanting bath. The tannic acid
has the property of combining with the basic constituents of these dyes
and forms a true colour lake, which is firmly fixed on the fibre. The
colour lake can be formed independently of the fibre by bringing the
tannic acid and the dye into contact with one another.

In the case of the dyes of the third group, the formation of a colour
lake between the metallic oxide and the colouring matter can be readily
demonstrated. In dyeing with these colours the cotton is first of all
impregnated with the mordanting oxide, and afterwards placed in the
dye-bath, the mordant already fixed on the fibre then reacts with the
dye, and absorbs it, thus dyeing the cot