246 THE POPULAR SCIENCE MONTHLY. 



The devices that have been used and are now employed for filtering 

 purposes are very numerous. Tracing their history from the old Hip- 

 pocratic 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 composi- 

 tion consisting of a mixture of charcoal, iron, and clay ; silicated car- 

 bon ; ground slag, or compounds of two or more of these substances 

 mentioned. 



The essentials of a good filter for domestic purposes are — 1. Effi- 

 ciency 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 possi- 

 bility 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 quali- 

 ties. 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 thor- 

 oughly 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 accom- 

 plish 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, w^hich 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." 



