and as a rule the indigo blues dyed in it are faster to rubbing.

After a day's work it can be well stirred up and fresh additions of
lime, zinc and indigo made to bring it up to its original dyeing
strength.

=(3) Zinc-Bisulphite Indigo Vat.=--When zinc dust and bisulphite of soda
are mixed together a reaction sets in, the zinc dissolves, and there is
formed sodium hydrosulphite and zinc and sodium sulphites. If now indigo
is mixed with this solution the sodium hydrosulphite exerts a reducing
action on it, forming white indigo and sodium sulphite, a perfectly
clear solution being obtained, which may be used in dyeing cotton or
wool.

With this vat it is customary to prepare a strong stock solution of
reduced indigo, and to add this to the dyeing vats as may be required.

_To Make the Stock Liquor._--Take 20 lb. of indigo, grind into a paste
with 20 gallons of boiled water, then add 25 lb. lime slaked into a
milk. In a separate tub there is mixed 80 lb. bisulphite of soda, 70 deg.
Tw., with 9 lb. zinc dust; this mixture is well stirred and every care
taken to prevent it getting hot. When the zinc has dissolved and the
mixture is free from any sulphurous smell it is run into the indigo
mixture given above. The whole is well stirred together for some time,
and then at intervals, until the indigo has become dissolved, sufficient
water is added to make up 50 gallons, when the stock liquor will be
ready. It should have a deep yellow colour. The surface may have a scum
of a bronzy colour collect on it. This stock liquor should be kept in
casks free from exposure to the air.

To make the working vat from this stock liquor the following is the
method of proceeding:--

Water is run into the vat, and this is heated from 70 deg. to 80 deg. C. in
order to expel air from it, after which it may be allowed to cool, then
for each 1,000 gallons contained in the vat there is added 30 lb.
bisulphite of soda, 3 lb. zinc dust and 3 lb. lime, made into a cream.
When all these ingredients are dissolved a quantity of the stock liquor
is added in proportion to the shade that it is desired to dye. The whole
is well stirred, then the vat is allowed to rest for half an hour to
enable any sediment to settle, and then the dyeing is proceeded with.

Should the vat show signs of becoming green in colour it is a good plan
to add a mixture of 1 lb. zinc dust and 10 lb. bisulphite of soda. The
vat should be kept alkaline, and so a little lime may