Note:  Do not rely on this information. It is very old.


Films. In photography, films, usually of sensitised gelatine, are frequently used in place of the ordinary coated-glass plates, for the production of negatives. Many different varieties of films are in use; paper is very frequently used as a support for the gelatine, either rendered transparent by vaseline, etc., or capable of being stripped off from the film after development as in the. Eastman stripping, film. They possess the advantages over glass of (1) being less liable to cause "halation," (2) possessing less weight, and (3) admitting of use in lengths or rollers. They are hence very popular amongst touring photographers, with whom portability is a great consideration. Filter is an instrument for the purification of water or other fluid. When water containing suspended particles of solid matter is passed through porous material such as powdered charcoal or unglazed earthenware, the passage of the solid matter is arrested and the liquid passes through in a state of greater purity. It is found also that a portion of those ingredients that are held in solution, and that therefore present no solid obstruction, is retained by the filtering medium. And further, organic impurity in the water may be oxidised and so destroyed by passage through a medium like charcoal, which possesses to a remarkable degree the power of absorbing air and of oxidising matter brought in contact therewith. On such principles are modern filters constructed, the chief practical difficulty being that of clogging; the pores of the filter are apt to become closed up by the impurities arrested, and the instrument ceases to act with sufficient rapidity. For the purification by filtering of large quantities of water such as are passed along the mains of a water company, filter-beds or reservoirs of special construction are used. The impure water as gathered in from the river or other source passes into a large basin and heavy sediment is allowed to settle down; the water is then passed into another basin the bottom of which is covered with successive layers of large stones, pebbles. gravel, and fine sand. In passing through these layers the water is well purified of suspended matter. Nearly the whole of the filtration occursin an extremely thin layer at the top of the filter bed, which rapidly becomes clogged with sediment and vegetable matter. At intervals of from one to three months a layer of about half an inch thickness of the fine sand is scraped away, this process being repeated till it becomes necessary to spread a fresh layer of sand over the filter-bed. The dirty sand is oxidised by exposure to the air, and after being well washed it may be again used. Filters such as are required for household purposes do not vary much in design. An efficient filter may be cheaply made with an ordinary flower-pot. The hole is blocked up with a piece of sponge, a layer of pebbles is placed at the bottom, and this is covered with a layer of fine gravel. Over this is spread powdered charcoal to a depth of four inches,. and this is protected by being covered with another layer of small pebbles. The charcoal requires occasional renewal, but the periods of such renewal are much farther apart if the water passes into the filter from another pot, the hole of which is plugged with sponge. Filters of ordinary manufacture are much the same in principle as the above. The charcoal is sometimes compressed into a solid, porous mass, but to no great advantage. The means adopted to prevent clogging are generally such as force the water to flow upwards through the filter, leaving the sediment at the lowest layers to fall back into the water below. Air is to a great extent purified of deleterious gaseous animal and vegetable products by being passed through a sieve filled with small fragments of charcoal. A sieve of fine muslin is found effective in purifying air of suspended solid particles of soot, etc., but it rapidly becomes filthy if used continuously in any large town.