completely as is the case with tartar emetic.

With copperas or iron liquor the question comes up not only of the mere
fixation of the tannin, but also the fact that iron forms with tannin
grey to black compounds, hence cotton which has been tanned and then
immersed in a bath of copperas becomes coloured grey to black, according
to the quantity of the tannin matter used. The property is useful when
dark shades of maroons, clarets, greens, browns, blues, etc., are to be
dyed, and is frequently employed.

(3) =Dyeing.=--After the tannin and fixing operations comes the dyeing.
This is not by any means a difficult operation. It is best carried out
by preparing a cold bath, entering the prepared or mordanted cotton in
this and heating slowly up. It is not necessary to raise to the boil nor
to maintain the dye-bath at that heat, a temperature of 180 deg. F. being
quite sufficient in dyeing with the basic colours, and the operation
should last only until the colouring matter is extracted from the
dye-bath. Working in this way, level uniform shades can be got.

One source of trouble in the dyeing of basic dyes, more especially with
magentas, violets and greens, lies in their slight solubility and great
strength. In preparing solutions of such dyes it is best to dissolve the
dye-stuff by pouring boiling water over it, and stirring well until all
appears to be dissolved.

This solution should be strained through a fine cloth, as any
undissolved specks will be sure to fix themselves on the cloth and lead
to dark spots and stains, as, owing to the weak solubility of the dye,
and this being also fixed as insoluble tannate by the tannic acid on the
fibre, there is no tendency for the dye to diffuse itself over the
cloth, as occasionally happens in other methods of dyeing. No advantage
is gained by adding to the dye-bath such substances as common salt or
Glauber's salt.

Some few of the dyes, _e.g._, Soluble blue, Victoria blue, which will
dye on a tannic mordant, are sulphonated compounds of the colour base.
These can be dyed in medium or light shades on to unmordanted cotton
from a bath containing alum, and in the ordinary method of dyeing the
addition of alum is beneficial, as tending to result in the production
of deep shades. These are somewhat faster to washing and soaping, which
is owing to the dye-stuff combining with the alumina in the alum to form
an insoluble colour lake of the sulphuric acid groups it contains.

Man