; in some respects this is the most economical plan,
only the dyer has to get rid of the spent dye-wood from which the
colouring matter has been extracted, and this is not always an easy
matter. Some dyeing machines (Smithson's) have been devised which
contain as one of their features a dye-wood extractor, in which the
extraction of the colouring matter of the wood proceeds at the same
time as the dyeing. Good results are got with such machines, although
they leave something to be desired.

Many dyers use the dye-wood extracts which are now made on a large
scale. These are for the dyer much more convenient to use, although
naturally rather more costly. They are approximately five times the
strength of the dye-wood, but they vary very greatly in this respect.

Logwood blacks can be readily distinguished from nearly all other
blacks, in that by treatment with moderately strong hydrochloric acid
they turn a bright red.

No other natural dye-stuff is used in the dyeing of black than these
here given.

Of late years many black dyes derived from coal tar have been (p. 089)
placed on the market. Among these may be enumerated the Acid Blacks of
Messrs. Bead Holliday & Sons; the Naphthol and Naphthylamine Blacks of
Leopold Cassella & Co.; the Victoria Blacks of the Farbenfabriken
vorm, Fr. Bayer & Co.; the Wool Blacks of the Actiengesellschaft fuer
Anilin Fabrikation; the Azo Blacks of the Farbwerke vorm, Meister,
Lucius & Bruning; and one or two other blacks. These blacks are dyed
very simply, as will be seen from the recipes given below, showing
their application in the production of blacks of a great variety of
tone. None of them dye a true jet black, but generally a bluish black
or a violet black, but the tone may be readily changed to a jet or
dead black by the addition of a little orange, yellow or green
dye-stuff.

They give blacks of a very solid appearance and very bright in tone,
and have the advantage over the logwood blacks of leaving the wool
more supple and less liable to be felted. Moreover, as a rule they are
faster to acids, alkalies and milling than are the logwood blacks, and
as regards fastness to light they excel that dye-stuff. Unfortunately
they are more costly to use, which tells against their entirely
displacing logwood in dyeing blacks on wool.

Still, year by year their use is increasing, and as their price
becomes less their employment will yet further extend. They may be
combined with lo