paper works, bleach works, etc. If the paper works be those working up
wood pulp, the pollutions of effluent water will be about as noxious as
they well can be. You will have gums and resins from the wood, calcium
chloride from the bleach vats, acids from the "sours"; resin, and
resin-soaps; there may also be alumina salts present. Now alumina, lime,
resin, and resin-soaps, etc., precipitate dyestuffs, and also soap; if
the water is alkaline, some of the mordants used may be precipitated and
wasted, and very considerable damage done.

Permanent hardness in water, due to the presence of gypsum or sulphate
of lime in solution, may be remedied by addition of caustic soda. Of
course, if an alkaline water is objectionable in any process, the alkali
would have to be neutralised by the addition of some acid. For use in
boilers, water might thus be treated, but it would become costly if
large quantities required such treatment. Water rendered impure by
contaminations from dyehouses and some chemical works can be best
purified, and most cheaply, by simple liming, followed by a settling
process. If space is limited and much water is required, instead of the
settling reservoirs, filtering beds of coke, sand, etc., may be used.
The lime used neutralises acids in the contaminated and impure water,
precipitates colouring matters, mordants, soap, albuminous matters, etc.

_Tests of Purity._--I will now describe a few tests that may be of value
to you in deciding as to what substances are contaminating any impure
waters that may be at hand.

_Iron._--If to a water you suspect to be hard from presence of carbonate
of lime or carbonate of iron in solution in carbonic acid, _i.e._ as
bicarbonates, you add some clear lime-water, and a white precipitate is
produced, you have a proof of carbonate of lime--hardness. If the