Contamination of Water by Factories.—­You may have neighbours higher up the stream than yourselves, and these firms may cast forth as waste products substances which will cause immense waste and loss.  Amongst these waste products the worst are those coming from chemical works, 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 precipitate is brownish, you may have, also, carbonate of iron.  I will now mention a very delicate test for iron.  Such a test would be useful in confirmation.  If a very dilute solution of such iron water be treated with a drop or two of pure hydrochloric acid, and a drop or so of permanganate of potash solution or of Condy’s fluid, and after that a few drops of yellow prussiate of potash solution be added, then a blue colour (Prussian blue), either at once or after standing a few hours, proves the presence of iron.

Copper.—­Sometimes, as in the neighbourhood of copper mines or of some copper pyrites deposits, a water may be contaminated with small quantities of copper.  The yellow prussiate once more forms a good test, but to ensure the absence of free mineral acids, it is first well to add a little acetate of soda solution.  A drop or two of the prussiate solution then gives a brown colour, even if but traces of copper are present.