Among the “Congo oranges and yellows,” we find some of the fastest on cotton of this class of colors.  Still they deserve only the rank of medium fastness.  They are Mikado orange 4 R, R, G. Hessian yellow, curcumin S, chrysophenin.  On wool, we have about half a dozen of medium fastness, viz., benzo-orange, Congo orange R, chrysophenin G, chrysamin R, brilliant yellow.  On silk, however, we find in this group about a dozen of the fastest oranges and yellows with which we are acquainted for this fiber, viz., Congo orange R, chrysophenin G, diamine yellow N, brilliant yellow, curcumin W, benzo orange, Hessian yellow, chrysamin R and G, cresotin yellow R and G, cotton yellow G, and carbazol yellow.

Does it not appear somewhat remarkable that we should find among this generally fugitive group of coloring matters colors which are so eminently fast on silk, and which we entirely fail to meet with among those groups which usually furnish our fast colors, e.g., the alizarin group?

Passing on to the “Congo violets, blues, and purples,” we find few colors worthy of particular notice for fastness.  Diamine violet N appears, perhaps, of medium fastness on wool and silk, while sulphonazurin, benzo-black blue, and direct gray may claim the same distinction on silk.

In the small group of colors which are produced directly upon the fiber, none seems to call for special notice, except aniline black, which, notwithstanding its direct derivation from aniline, is probably the fastest color we have upon any fiber.

Now, in classifying the whole range of coal tar coloring matters into “mordant dyes” and “direct dyes,” and the latter into acid, basic, Congo colors, etc., I have looked at them from the point of view of the dyer and arranged them according to color and mode of application.  The chemist, however, classifies them quite differently, viz., according to their chemical constitution, i.e., the arrangement of the atoms of which they are composed, and thus we have nitro colors, phthaleins, azines, and so on.

In studying the action of light on the coal tar colors from this point of view, we find that whereas the members of some groups are for the most part fugitive, the members of other groups are nearly all fast, and it becomes at once apparent that the chemical constitution of a coloring matter exercises a profound influence upon its behavior toward light.  Members of the rosaniline group are all similarly fugitive, while those of the alizarin group possess generally the quality of fastness.  Particularly fugitive are the eosins, and yet some of these, by a slight modification of constitution, e.g., the introduction of an ethyl group, as in ethyl-eosin, are rendered distinctly faster.

In the azo group some colors are fugitive, others are moderately fast, and it is generally recognized that certain classes of the tetrazo compounds are distinctly faster than the ordinary diazo colors.