It is often of great advantage to render a filter "self-acting," or to construct it in such a way that it may "feed itself," so that it may continue full and at work without the constant attention of the operator. On the small scale, this may be readily effected on the principle of the common fountain lamp (see engraving); and on the large scale, by placing the vessel containing the un-filtered liquid on a higher level than the filter, and by having the end of the supply pipe fitted with a ball-cock, to keep the liquid in the filter constantly at the same height.

The rapidity of filtration depends upon the porosity of the filtering medium, the extent of the filtering surface, the relative viscidity or mobility of the filtering liquid, the pressure or force by which the liquid is impelled through the pores of the filter, and the porosity and fineness of the substances it holds in suspension. The most efficient filter is produced when the first two or the first three are so graduated to the .others that liquid filters rapidly, and is at the same time rendered perfectly transparent.

Fig. 378.   Self acting Filter

Fig. 378. - Self-acting Filter.

Fig. 379.   Pressure Filter

Fig. 379. - Pressure Filter.