nd this solution may be used
in dyeing. To prepare the hydrosulphite a vessel which is fitted (p. 142)
with an agitator and can be closed is filled with zinc, either in the
form of dust, foils, or granules. Then bisulphite of soda of 50 deg. to
60 deg. Tw. strength is poured over the zinc in sufficient quantity to
cover it. All access of air should be avoided as much as possible, as
it leads to oxidation. In the case of using zinc powder the action is
often so rapid as to lead to heating, which also should be avoided.
The operation takes from an hour to two hours, when the liquor may be
drawn off. It must be used immediately to dissolve the indigo; or
otherwise, as it is a very unstable body, it is liable to decompose
and become oxidised, when it loses its solvent properties. If more
hydrosulphite is required, fresh bisulphite may be poured over the
zinc which is left unused in the vessel; if no more is wanted the zinc
which is left should be well rinsed in water and the vessel filled
with water, so as to prevent any oxidation of the zinc, and so keep it
ready for use when required. The liquor thus made will usually have a
specific gravity of 62 deg. Tw. The zinc which is used up in the
preparation of the liquor is replaced by fresh zinc from time to time.

The liquor so obtained is, as stated above, rather unstable, and
contains acid sodium hydrosulphite. By mixing with milk of lime, the
acidity is neutralised, zinc oxide and calcium sulphite are thrown
down, and a solution of neutral sodium hydrosulphite is obtained which
is more stable and can be kept longer without decomposition. To
prepare this, take 10 gallons of the acid liquor, as prepared in the
manner described above, and mix it with 48 lb. of milk of lime, which
is made from 2 lb. good quick-lime. Stir well together, allow all
sediment to settle, or better, filter-press the mass. A liquor of
36 deg. Tw. strength will usually be obtained. Do not let it stand too
long before use, make it alkaline by adding a little lime.

To make the mother or stock indigo, the following method of (p. 143)
procedure may be adopted. Indigo, say 10 lb., is ground into as fine a
paste as possible with 13 lb. milk of lime, of such a strength that 1
gallon shall contain 30 oz. quick-lime. To this is then added so much
of either the acid or the neutral sodium hydrosulphite as can be made
from 90 lb. of bisulphite of soda, the mixture being kept at 150 deg. F.,
until a compa