Page:Popular Science Monthly Volume 25.djvu/256

246 The devices that have been used and are now employed for filtering purposes are very numerous. Tracing their history from the old Hippocratic sleeve, which was a cone-shaped bag of cotton or wool, we find, among others, the following materials: Thick unsized paper; cloth of various texture; sand; asbestus; animal charcoal; vegetable charcoal; felt; porous stones of various kinds; spongy iron; porous earthenware; perforated metallic disks; sponge; carferal, a composition consisting of a mixture of charcoal, iron, and clay; silicated carbon; ground slag, or compounds of two or more of these substances mentioned.

The essentials of a good filter for domestic purposes are—1. Efficiency in removing foreign bodies held in suspension. 2. Chemical power to destroy animal and vegetable impurities in solution or to convert them into innocuous substances. 3. Freedom from all possibility of tainting the water. 4. Simplicity of construction, so as to admit of the filtering material being readily renewed. 5. Cheapness. A good filter for domestic purposes must possess all five of these qualities. Those that have two or three of them and lack the remainder do not practically solve the problem of giving us clean water to drink.

The Japanese use a porous sandstone hollowed in the shape of an egg, through which the water percolates into a receptacle underneath; the Egyptians resort to a similar device; the Spaniards use a porous earthen pot. But these and other similar contrivances can not be thoroughly cleansed; after the most thorough rinsing, some impurities will remain in the pores of the stone. Spongy iron and carferal are open to the same objection; they will answer well for a short time, but soon become contaminated by pollution retained in their pores. Sponge, cloth, and felt, unless cleaned every day or two with hot water, will do more harm than good, and the average servant-girl will not clean them or any other filter unless under the eye of her mistress.

The various forms of filters that are screwed to the faucet have only to be hastily examined to be discarded, as there is not sufficient filtering material in them to be of much utility, and they very soon become foul and offensive. Buck says, "There is no material known which can be introduced into the small space of a tap-filter and accomplish any real purification of the water which passes through at the ordinary rate of flow."

The various complicated closed filters, filled with any material which can not be removed for cleansing, condemn themselves. No amount of pumping water through them at different angles, which is at all likely to be used, can cleanse them of the impurities that adhere to the mass and in the pores of the filtering material used. Parkes, in his "Manual of Practical Hygiene," says, "Filters, where the material is cemented up and can not be removed, ought to be abandoned altogether."