This is accomplished by the aid of permanganate of potash, After the presence of organic impurities has been ascertained by the tests given on another page, their combination with oxygen and subsequent precipitation is caused by the addition of some crystals or a solution of this chemical salt. Add about from 3 to 6 drachms of permanganate of potash crystals, or about one-half to an ounce of the solution of the salt, as prepared according to directions given on another page, to every fifty gallons in cistern or barrel containing the water to be purified, and stir lively with a wooden spatula, all to be done before filtering, and it will greatly assist in removing organic impurities. If the purple color it imparts to the water disappears rapidly, it is a proof that the water is very much contaminated with organic matter; add some more until the coloration ceases to disappear, and a slight purple hue is visible. When possible let the mixure rest for three days to give time for subsidence. Then filter through animal charcoal, which absorbs the balance of the color and turns the water out bright and clear.

Care must be taken not to use too much of the permanganate of potash, although it does no harm when used in excess. Still the animal charcoal in the filter, which will absorb the coloration, would at last leave the purple tint in the water when its activity is too soon exhausted.