happens that some dye does not combine with the fiber and mordant, thus
becoming fixed, but merely incrusts the fiber; hence this portion is
washed off when the retaining film of grease is removed from the fiber.
The suds, therefore, after fulfilling this purpose, are no longer a pure
solution of soap, but contain many foreign matters; and the problem is
so to treat these suds as to recover the fat in some condition available
for re-conversion into soap.

For this purpose wooden runnels are placed beneath the rollers, through
which the cloth passes in the scouring machine, so as to collect the
suds after they have been spent. These runnels lead to a wooden pipe or
runnel, which receives the spent suds from all the scouring machines,
and the whole of the waste, instead of being let off into the stream,
polluting it, delivers into a tank or trough, which may also be
constructed of wood, but, as it has to withstand the action of acid, is
better lined with lead. This tank is necessarily proportioned in size
to the number of scouring machines and the quantity of spent suds to
be treated. When a sufficient quantity has collected, oil of vitriol,
diluted with twice its bulk of water, is added, one workman pouring it
in gradually while another stirs the contents of the tank vigorously. At
short intervals, the liquid is tested by means of litmus paper, and
when it shows a faint acid reaction, by turning the blue paper red, the
addition of acid is stopped. The acid has then combined with the alkali
of the soap, while the fatty acids formerly in combination with the
alkali are liberated, and float to the surface of the liquid, carrying
with them the impurities in the shape of short fibers and dye stuffs;
the sand and heavier impurity, should any be present, sinks to the
bottom.

After standing for some hours, the separation is complete. In order to