Among the red dyes we have alizarin and its numerous allies, and these are certainly fit representatives of the madder root, which indeed they have almost entirely displaced.  The most recent additions to this important class are the various alizarin Bordeaux.  The only dyes in this group which appear somewhat behind the rest in point of fastness are purpurin and alizarin maroon.

On this same diagram we notice, also, fast blues and dark greens, of which we have no similar representatives among the natural coloring matters.  I refer to alizarin blue, alizarin cyanin, alizarin indigo, alizarin green, and coerulin.

Further, an excellent group of coloring matters, giving fast browns and greens with copper and iron mordants respectively, is formed by naphthol green, resorcinol green, gambin, and dioxin.

The only fugitive dyes of the class now under consideration are some of the yellows, gallamin blue and gallocyanin.

If we now turn to examine the colors given by these artificial “mordant dyes” on silk, we notice, also, a good series of fast colors similar to those which they give on wool; and even on cotton we see many fast colors, of which we have no representatives among the dyewoods.

If we were not prepared to find so few really fast natural dyes, surely we cannot but be surprised to find what a considerable number 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.