3}
(oxygen).] It is this hydrated chromic oxide in the fibre that yields
with the Haematein of the logwood your logwood black dye. Mr. Jarmain
finds that it is not safe to use more than 3 per cent. (of the weight of
the wool) of bichromate; if 4 per cent. be used, the colour becomes
impaired, whilst if 12 per cent. be employed, the wool cannot be dyed at
all with logwood, the phenomenon known as "over-chroming" being the
result of such excessive treatment. I think there is no doubt, as
Professor Hummel says, that the colouring matter is oxidised and
destroyed in such over-chroming, but I also think that there can be no
doubt that the wool itself is also greatly injured and incapacitated for
taking up colour. Now the use of certain coal-tar black dyes in place of
logwood obviates this use of bichrome, and thus the heavy stress on the
fibre in mordanting with it. It also effects economy in avoiding the use
of bichrome, as well as of copper salts; but even thus, of course, other
problems have to be solved before it can be finally decided which is
best.

LECTURE IX

DYESTUFFS AND COLOURS

_Classification._--In classifying the different dyestuffs and colouring
matters it is, of course, necessary to consider first the properties of
those colouring matters generally, and secondly the particular reason
for making such classification. The scientific chemist, for example,
would classify them according to theoretical considerations, as members
of certain typical groups; the representative of medical science or
hygiene would naturally classify them as poisonous and non-poisonous
bodies; whilst the dyer will as naturally seek to arrange them according
to their behaviour when applied to textile fabrics. But this behaviour
on applying to textile fibres, if varied in character according to the
chemical nature of the colouring matter, as well as the chemical and
physical nature of the fabric--and it is so varied--will make such
classification, if it is to be thorough-going, not a very simple matter.
I may tell you that it is not a simple matter, and, moreover, the best
classification and arrangement is that one which depends both on the
action of the dyes on the fibres, and also on the intrinsic chemical
character of the dyestuffs themselves. Since the higher branches of
organic chemistry are involved in the consideration of the structure and
dispositions, and consequently more or less of the properties of these
dyes, you wi