FILTER, in chemistry, a strainer com monly made of bibulous or filtering paper in the form of a funnel, through which any fluid is passed, in order to separate the gross particles from it, and render it limpid. There are several filters made of flannel and linen cloth. The filter pro duces the same effect, with regard to li quids, that the sieve does in dry matters. Filters are of two sorts: the first are sim ple pieces of paper or cloth, through which the liquor is passed without farther trouble ; the second are twisted up like a skein or wick, and first wetted, and then squeezed as dry as possible ; one end is. put into the liquor to be filtrated, the other end is to hang out below the- sur face of the liquor ; by this means the purest part of the liquor distils drop by drop out of the vessel, leaving the dregs behind : a filter of this kind acts upon the principle of the syphon. Water is freed from various impurities by means of ba sins made of porous stone ; this is often very necessary at sea, when the water be comes foul, and on land, where there are no fresh springs. The filter is of use to all those in and near the metropolis, who are supplied with water from the Thames, the New River, and the ponds from Hampstead. Many patents have been obtained for filtering machines, which may be seen in various parts of London.
We shall observe, that Mr. Peacock obtained, about twelve years since, one for a new species of filtration, by means of gravel of different sizes, suitable to the several strata. The various sizes of the particles of gravel, as placed in layers, should be nearly in the quadruple ratio of their surfaces ; that is, upon the first layer, a second isto be placed, the diame ters of whose particles are not to be less than one half of the first, and so on in this proportion. This arrangement of filter ing particles will gradually fine the water, by the grosser particles being quite inter cepted in their partly ascending with the water. An advantage in these filters is, that they may be readily cleansed by drawing out the body of the fluid, by which it will descend in the filter, and carry with it all the foul and extraneous substances.
A patent was also granted to Mr. Joshua Collier of Southwark, for a most ingeni ous method of filtering and sweetening water, oil, and every other liquid. The following is the contrivance, which com bines the application- of machinery with the antiseptic properties of charcoal. Fish oil is one of the liquids which he bad particularly in view, to free it from every thing disagreeable, either in taste, smell, or colour ; to accomplish which lie poured a quantity of oil into a conveni ent vessel, heated to the temperature of 120° of Fahrenheit's thermometer, add ing caustic mineral alkali of the specific gravity of 1.25. He then agitated the mixture, afterwards allowing it to stand till the sediment subsided, and then drew it off into another vessel, with a suffici ent quantity of burnt charcoal finely powdered, and a small quantity of diluted sulphuric acid, to decompose the sapo naceous matter still suspended in the oil, when the oil became clear at the surface he then agitated the contents of this vessel, and left the coally, saline, and aqueous particles to subside ; after wards passing it through proper strainers, when it became quite transparent and fit for use.
The principle of the improved filter ing machines consists in combining hy drostatic pressure with the mode of fil tering per ascension, which procures the peculiar advantage of causing the fluid and its sediment to take opposite direc tions. The filtering surface remains the same, while the dimensions of the cham ber in which the sediment is received may be varied. To adapt the machines to every purpose for which they are in tended, chambers must be provided, of various capacities, for the precipitated matter. The space required is very great with respect to the oil trade, and as all dimensions will be required oc casionally, no particular limits can be fixed. For distilleries and breweries they may be smaller in proportion, and a very small chamber will he sufficient for domestic economy. If water is to be freed from noxious particles, it must be made to pass through all iron box in its way to the filtering chamber, and the box most contain charcoal finely pow dered; the water is received into this box, and delivered by two apertures, which are opened and closed by cocks. Another part of the invention consists ha filtering machines in the form of stills, in which charcoal may be repeatedly burnt, after any fluid substances have passed through it, for the purpose of freeing them from noxious particles, or discharg ing their colouring matter.
To the filtering apparatus of Mr. Col lier, instruments are attached for disco vering the comparative qualities of oils, which depend, in some measure, on their specific gravities : spermaceti oil, when compared with fish oils, being as 875 to 920. To do this a glass vessel, of any shape most convenient, is employed, with a glass bubble and a thermometer. If is pure, the bubble sinks, when the mercury rises to a particular stand. ard. When spermaceti oil is impure, the bubble floats, though of the tempe rature required. To determine the ten dency of oils, used for burning, to con. geal in cold weather, a freezing mix ture may he put into a phial of thin glass, into which let a thermometer be immersed, and a single drop of the oil permitted to fall on the outside of the vessel, where it will instantly congeal_ As the cold produced by the mixture de creases, let the temperature be observed, by the thermometer, at which the oil be comes fluid, and runs down the side of the glass.
FIN, in natural history, a well known part of fishes, consisting of a membrane supported by rays, or little bony or car tilaginous ossicles.
The number, situation, and figure of fins, are different in different fishes. As to number, they are found from one to ten, or more ; with -respect to situation, they are dorsal, placed on the back ; pectoral, generally situated on the side near the gills ; ventral, on the belly be fore the vent ; anal, when behind the vent ; and caudal, terminating the tajl ; and as to figure, they are either of a tri angular, roundish, or oblong square form. Add to this, that in some they are very small ; whereas in others they are almost equal to the whole body in length.