and already the number of coal tar colors which have been discarded,
for one reason or another, is considerable.

Not unfrequently one is asked the question, Is there no method whereby
these fugitive colors can be made fast? Knowing the efficacy of
mordants with certain coloring matters, is there no mordant which we
can generally apply with this desirable object in view? The discovery
of such a universal mordant I believe to be somewhat chimerical, and
yet, curiously enough, a number of experiments have been recorded in
recent years, which almost seem to point in the direction of selecting
for such a purpose ordinary sulphate of copper.

Some of these diagrams before you this evening show clearly the
fastness to light generally of the lakes formed with copper mordant.
This peculiarity of the copper compounds has not escaped the notice of
other observers. Dr. Schunck, for example, during the progress of his
research on chlorophyl, noticed the very permanent green dye which
this otherwise fugitive coloring matter gives in combination with
copper.

Then there is the assertion of practical dyers, that the use of copper
sulphate in dyeing catechu brown on cotton assists materially in
rendering this color fast to light.

The use of copper mordant with phenolic coloring matters is perfectly
natural. Some time ago, however, it was successfully applied, for the
purpose of rendering more permanent, to certain of the Congo colors on
cotton, e.g., benzo-azurine, etc., in the application of which,
metallic salts had not hitherto been deemed necessary.