sunk to the bottom is removed, dried, and forms a valuable manure,
owing to the amount of the nitrogen which it contains. Its value may be
increased by addition of weak vitriol, which exercises a decomposing
action on the nitrogenous matter, forming with it sulphate of ammonia.
The original residue from the filter-press, if it does not contain
indigo, may be at once put to similar use.

In large works, which dye their own goods, it is well known that the
"fermentation vat" is in general use for indigo-dyeing. But this vat
requires constant superintendence, and must be kept in continual action;
besides, it is successful only on a comparatively large scale. And,
moreover, it requires skilled labor. Small works, or works in which
dyeing is only occasionally practiced, find it more convenient to use
Schuetzenberger and Lalande's process. Although this process is well
known, a short description of it may not here be out of place.

The process depends on the reduction of indigo to indigo-white, or
soluble indigo, by means of hyposulphite, or, as it is generally termed
to avoid confusion with antichlore, rightly named thiosulphate of soda,
hydrosulphite of soda. The formula of this substance is NaHSO_{2}, as
distinguished from what is commonly known as hyposulphite of soda,
Na_{2}S_{2}O_{3}. It is produced by the action of zinc-dust on the acid
sulphite of soda. The zinc may be supposed to remove oxygen from the
acid sulphite, NaHSO_{3}, giving hyposulphite, NaHS0_{2}. The reduction
of the acid sulphite is best performed in a cask, which can be closed at
the top, so as to avoid entrance of air. The acid sulphite of soda, at a
strength of 50 or 60 Twaddell (specific gravity 1.26 to 1.3), is placed
in the cask, and zinc-dust is added, with frequent stirring. The liquid
is then mixed with milk of lime, and after again thoroughly stirring,
the liquid is allowed to settle, and the clear is decanted into the