Page:The American Cyclopædia (1879) Volume VII.djvu/197

 FILTER 189 Mr. Buchanan or Mr. Fremont, believing that there was no hope of Mr. Fillmore's election ; and though he received the support of large numbers in all the states, Maryland alone gave him its electoral vote. Mr. Fillmore afterward resided in Buffalo, taking no prominent part in public affairs. FILTER, an apparatus for separating from fluids the foreign substances mechanically in- termixed with them. Beds of sand and gravel constitute natural filters, through which the surface water from rain percolates and may be collected in wells, or through which springs may pass upward from substrata. Artificial filters are constructed upon the same principle ; a diaphragm of some substance is presented, through the pores of which the fluid can pene- trate, but which are so fine that they arrest the particles held in suspension. They are variously constructed according to the nature of the fluid to be purified. The chemist takes a disk of thick unsized paper prepared for the purpose, doubles it twice, introduces it into a funnel of proper size,' which, for facilitating the passage of the fluid, is commonly ribbed, opens one of the folds, and pours in the liquid. The fluid portion passes through the paper, leaving all the solid particles upon the filter. Filters used by chemists and apothecaries are usually made of paper, but felt, cloth, and cot- ton, woven and unwoven, are often used, as also layers of charcoal, sand, asbestus, and similar materials. The old pharmaceutists used a cone-shaped bag of cotton or woollen called Hypocrates's sleeve, and the same contrivance is still considered one of the best for the clari- fication of sirups and other viscid liquids. The. conical hat body made of felt is well adapted, before its shape is altered, to the filtration of fixed oils. Corrosive liquids, as strong acids, may be cleared by passing them through pure silicious sand supported upon coarse fragments of glass placed in the neck of a funnel and gradually diminishing in size upward. Char- coal is a favorite material, particularly for the purification of water used at sea ; it has the property not only of separating the impurities passed through layers of it alternating with others of sand, but also of removing disagree- able odors. The Japanese use porous sand- stone hollowed into the form of an egg, and set in a frame over a vessel, into which the water drops as it percolates through the stone. The Egyptians adopt the same method for clarifying the water of the Nile. A stone which answers this purpose well has long been known at Teneriffe, and was formerly largely imported into England. In Spain porous earth- enware vessels are manufactured, called alcar- /";v.s', which are used for this purpose, and also for wine-coolers. An ingenious filter was noticed at the London international exhibition of 1851, sent by the Wenham lake ice company of Massachusetts, the invention of Mr. Alfred Bird. It consists of a siphon, the short limb of which terminates in a cylindrical box, which is placed in a cask of water under the surface. This box contains the filter, and on drawing the air out of the long arm, which projects from the cask, the water is forced up through the filter and passes through the siphon, its flow being regulated by a stopcock at the lower end of the long arm. It has the advantage, if the cask is kept properly supplied, of drawing the water neither from the top where the light- er sediment collects, nor from the bottom to which the heavier impurities sink. Filters upon a large scale are connected with the res- ervoirs from which cities receive supplies of water. These reservoirs are divided into sev- eral basins, the first of which are intended for receiving the sediment that will subside as the water is left standing; from these it passes through porous beds which separate them from an adjoining basin, and which collect the im- purities still remaining suspended. By using several such basins the beds may be kept al- ternately in use, affording an opportunity for cleansing them whenever this is required. In many large reservoirs, as the Croton, no filter- ing apparatus is used, the water passing from the first to the second, and even to the third and fourth, and the separation of clear water and sediment being effected by precipitation. The water of such reservoirs is often filtered by attaching diaphragms to the delivering pipes in the houses. In connection with the purifi- cation of water by filtration, ingenious meth- ods have been devised of separating the soluble salts of lime, which give the property of hard- ness to water, and which being in the state of solution pass through the filter. Pure water can hold only about two grains to the gallon of carbonate of lime, or ^,-0-0 ; tmt; as the water absorbs carbonic acid gas, its power of dissolving carbonate of lime increases till its capacity may be ten times that of pure water. Its hardness increases with the quantity of lime taken up. Thus the water of springs, especially in districts where calcareous rocks abound, differs in composition from the soft rain water which has not flowed through the ground. When such water is boiled, the ex- cess of carbonic gas is expelled, and with it the capacity of holding a portion of the carbo- nate of lime. This falls as a precipitate, and forms the crust which collects on the inside of kettles in which such wa"ter is boiled. By continuing the boiling, all the lime may be thus separated, except about two grains to the gallon ; and it is then in the best condition to be purified by filtering. Other salts, the sol- ubility of which does not depend upon the carbonic acid gas present, can only be separated by distillation. Some substances often present in the state of suspension in water cannot be separated either by subsidence or filtration ; such are some organic matters, and the fine clayey or aluminous particles. Waters which wash cliffs of clay become saturated with the impalpable material, which they almost wholly refuse to shed by any mechanical action. This