of fast dyes are to be met with among the coal tar coloring matters
requiring the aid of mordants.

On these diagrams, the first vertical column shows the stain given by
the coloring matter alone; the remaining columns show the colors
obtained when the same coloring matters are applied in conjunction
with the several mordants--chromium, aluminum, tin, copper, and iron.

It was formerly held that the office of a mordant was merely to fix
the coloring matter upon the fiber; we now know, however, and it is
plainly illustrated by these diagrams, that this view is erroneous,
for the mordant not only fixes but also develops the color; the
mordant and coloring matter chemically combine with each other, and
the resultant compound represents the really useful pigment or dye. If
a coloring matter is combined with different mordants, the dyes thus
obtained represent distinct chemical products, and it is quite
natural, therefore, to find them differing from each other in color,
and their resistance toward light.

Knowing this, it is clearly the duty of the dyer to apply each
coloring matter of this class with a variety of mordants, and to
select the particular combination which gives him the desired color
and fastness. By adopting this method, however, his selection would
ultimately comprise a large number of coloring matters paired with a
great variety of mordants. In order, therefore, to avoid the intricacy
involved in the use of several mordants, and to simplify the process
of dyeing, especially when dyeing compound shades, the dyer prefers to
limit himself as far as possible to the use of a single mordant, and
to employ along with it a mixture of several coloring matters.