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.

By a careful study of the influence of the atomic arrangement upon the
stability of colors, information useful to the color manufacturer may
possibly be gained, but at present my facts are not yet sufficiently
tabulated to enable one to recognize any generally pervading law in
this direction.

It is scarcely necessary to say that the fastness to light of a color
is independent of its commercial value, this being mainly determined
by the price of the raw material from which it is manufactured, the
working expenses, and the profit desired by the manufacturer. Neither
must we suppose that facility of application necessarily interferes
with its fastness to light, for some of our fastest coal tar colors on
wool, e.g., diamine fast red, tartrazin, etc., are applied in the
simplest possible manner. On the other hand, the intensity or depth of
a color has considerable influence on its fastness. Dark full shades
invariably appear faster than pale ones produced from the same
coloring matter, simply because of the larger body of pigment present.
A pale shade of even a very fast color like indigo will fade with
comparative rapidity. The fugitive character of many of the coal tar
colors is, in my opinion, rendered more marked, because, owing to
their intense coloring power, there is often such an infinitesimal
amount of coloring matter on the dyed fiber. Hence it is that in the