Corrosive liquids, as the strong acids, are filtered through powdered, glass, or so-called glasswool, or siliceous sand, supported on pebbles in the throat of a glass funnel, or through asbestos or gun-cotton placed in the same manner. Charcoal has also been employed for the same purpose, but is not fit for some acids. Strong caustic alkaline lyes are also filtered through powdered glass or sand. Weak alkaline lyes may be filtered through fine calico, stretched across the mouth of a funnel. Many corrosive liquids, as solution of potassa, etc., require to be excluded from the air during filtration. The simplest apparatus that can be employed for this purpose is that figured in the engraving annexed: - a is a globular bottle fitted with the ground stopper d, and having a perforated neck, f, ground to the bottle, b; c is a small tube, wrapped round with as much asbestos, linen, or calico, as is required to make it fit the under neck of the bottle through which it passes. The tube c may also be fixed by placing pebbles and powdered glass or sand around it, as before mentioned. For use, the solution to be filtered is poured into the bottle a, nearly as high as the top of the tube c, and the stopper is replaced. The liquid then descends into b, and a similar quantity of air passes up the tube into a. Liquor potassce may be always obtained fine by depuration in close vessels, when the sediment of lime only need be filtered, which may be effected with calico fixed across the mouth of a funnel. (See Fig. 377.) gaining Precipitates. - When a precipitate, or the suspended matter in a liquid, is the object of the filtration, the filter should be of such a nature that the powder may be easily separated from it, when dry, and that with the least loss possible. Linen filters are for this reason preferable for large quantities, and those of smooth bibulous paper for small ones. The powder should be washed down the sides of the filter, and collected, by means of a small stream of water, in one spot at the bottom, assisting the operation with a camel-hair pencil; and, when the whole is dry, it should be swept off the paper or cloth with a similar pencil or brush, and not removed by a knife, as is commonly done, when it can be possibly avoided.