the swatches should be well washed with water,
when the original colour may be wholly or partially restored.

To give tables showing the effects of these reagents on the numerous
dyes now known would take up too much room and not serve a very useful
purpose, as such tables if too much relied on leave the operator
somewhat uncertain as to what he has before him. The reader will find
in Hurst's _Dictionary of Coal-Tar Colours_ some useful notes as to
the action of acids and alkalies on the various colours that may be of
service to him.

Alizarine and the series of dye-stuffs to which it has given its name,
fustic, cochineal, logwood and other dyes of a similar class, require
the fabric to be mordanted, and the presence of such mordant is
occasionally an indirect proof of the presence of these dyes.

To detect these mordants a piece of the swatch should be burnt in a
porcelain or platinum crucible over a bunsen burner, care being taken
that all carbonaceous matter be burnt off. A white ash will indicate
the presence of alumina mordants, red ash that of iron mordants, and a
greenish ash chrome mordants.

To confirm these the following chemical tests may be applied. Boil the
ash left in the crucible with a little strong hydrochloric acid and
dilute with water. Pass a current of sulphuretted hydrogen gas through
the solution, if there be any tin present a brown precipitate of tin
sulphide will be obtained. This can be filtered off. The filtrate is
boiled for a short time with nitric acid, and ammonia is added to the
solution when alumina is thrown down as a white, gelatinous precipitate,
iron is thrown down as a brown red, bulky precipitate, while (p. 221)
chrome is thrown down as a greyish-looking, gelatinous precipitate.
The precipitate obtained with the ammonia is filtered off and a drop
of ammonium sulphide added, when any zinc present will be thrown down
as white precipitate of zinc sulphide; to the filtrate from this
ammonium oxalate may be added, when if lime is present a white
precipitate of calcium oxalate is obtained.

A test for iron is to dissolve some of the ash in a little
hydrochloric acid and add a few drops of potassium ferrocyanide
solution, when if any iron be present a blue precipitate will be
obtained.

To make more certain of the presence of chrome, heat a little of the
ash of the cloth with caustic soda and chlorate of soda in a porcelain
crucible until well fused, then dissolve in water, a