(p. 150)
plaited down on a table. The illustration clearly shows the working of
the machine.

#Dyeing Wool with Indigo Extract.#--Sulphonated indigo, prepared by
dissolving indigo in sulphuric acid, is sold under the name of "indigo
extract," or "indigo carmine," in two forms--paste (containing,
perhaps, 25 to 30 per cent. actual colour) and powder. Both forms are
freely soluble in water, although some makes are more so than others.
This quality of solubility is dependent upon the proportion of
sulphuric acid which may have been used in the preparation of the
extract. When this is small, what is termed indigo monosulphonic acid
only is formed, which is but slightly soluble in water, and gives red
shades. If a larger proportion of acid be used, then the indigo
disulphonic acid is formed, which is fairly easily soluble in water,
and gives bluer shades than the former.

As all forms of indigo extract are regular articles of commerce,
details for their preparation will not be given here. It will suffice
to say that indigo is heated with strong sulphuric acid until test
samples show that the indigo has been completely dissolved, and it is
then diluted with water and filtered. Sometimes it is sold in this
condition under the term "chemic," but if this be used in dyeing wool
it gives rather unsatisfactory results. When "sour extract" is
required, the liquor filtered out is next treated with salt until all
the colour has been precipitated out, when it is filtered off,
drained, pressed and sold. Should "neutral" or "sweet" extract be
required, then the acid liquor is neutralised with soda, and the
product is salted out as before, drained and pressed to a suitable
consistence. It is then sold as "indigo extract," or dried, at
150 deg. F., to a powder, which is known as "indigo carmine".

All forms of indigo extract are dyed on wool from baths of (p. 151)
Glauber's salt and sulphuric acid, and therefore they can be classed
with the acid-dyeing coal-tar colours. Indigo extract is notable for
its level dyeing and penetrative properties, but it is not fast to
light or milling.

Messrs. Read Holliday & Sons have a powder form of indigo extract
which will be found very useful and to give better shades than the
usual run of paste extract, while it only takes about one-fifth the
quantity to give a similar shade. Working at the boil should be
avoided with indigo extract, as tending to make the shades greenish in
tone;