402 IDEAL HOME LIFE
the substances from which most harm is likely to be derived. The water which contains them may be as clear as crystal, and yet it may abound in deadly poisons. It is not well to trust therefore to the appearance of water in deciding its potability and healthfulness.
How may water which is of doubtful purity be rendered harmless ?
It is not always easy to change one's water supply; it is always possible to render it absolutely free from harmful influences Among the methods which are in use for the artificial purification of water may be mentioned filtration, the use of chemicals, boiling, and distillation.
Water which is simply turbid, without materials which are actually injurious, a condition which is of usual occurrence after a storm or other exciting cause which disturbs the bottom over which it flows, may be rendered clear by simply allowing it to stand for a few minutes before it is used. The substances which are in suspension then sink to the bottom of the containing vessel, the clear fluid can be poured off, decanted, and the lower portion containing the sediment can be discarded.
Filtration is a very common method of treating water which is impure. Sometimes it is effective, and often it is ineffective. It consists in passing the water through a porous substance, the impurities being left behind if possible. Sometimes a porous stone, soapstone, porcelain, etc., is used; sometimes sand, charcoal, or other substances are used as percolators, the theory being that the pure water filters through and leaves the impurities behind. This may be effective for large particles; it is not effective for substances which are in solution. Besides, if the filtering material is not frequently renewed, the pressure of the water may be sufficient to force through the material which has accumulated from the first nitrations, and the filter becomes of no use whatever. Many forms of filtering apparatus are on the market; the most of them are worthless so far as furnishing protection from water impurities is concerned. A piece of coarse linen toweling wound about the water tap,