In reference to funnels, it may be remarked that those employed for filtering rapidly should be deeply ribbed, best spirally, on the inside, or small rods of wood or glass, or pieces of straw, or quills, should be placed between them and the paper. The neck or tubular part of the funnel should, in like manner, be deeply ribbed or fluted on the outside, to permit of the free passage of the air, when it is placed in a narrow-mouthed bottle or receiver. When this is not the case, filtration proceeds but slowly, and the filtered liquid is apt to be driven up the outside of the neck of the funnel by the confined air, and to be continually hissing and flowing over the mouth of the vessel. The breadth of a funnel, to filter well, should be about three-fourths its height, reckoning from the throat (a). When deeper, the paper is liable to be continually ruptured, from the pressure of the superincumbent fluid; and when shallower, filtration proceeds slowly, and an unnecessarily large surface of the liquid is exposed to the atmosphere and is lost by evaporation To lessen this as much as possible, the upper edge of the glass is fre quently ground perfectly smooth, and a piece of smooth plate-glass is laid thereon. When paper filters are of large dimensions, or employed for aqueous fluids that rapidly soften the texture of the paper, or for collecting heavy powders, or metallic precipitates, it is usual to support them on linen or calico, to prevent them breaking. This is best done by folding the cloth up with the paper, and cutting the filter out of the two, in the same way as would be done with double paper, observing so to place it in the funnel that the paper and calico may remain close together, especially towards the bottom.

Fig. 360.   Plaited Paper Filter

Fig. 360. - Plaited Paper Filter.

Fig. 361.   Filtering Paper

Fig. 361. - Filtering Paper.

The filtration of small quantities of liquid, as in chemical experiments, ma) often be conveniently performed by merely placing the paper on the circular top of a recipient (see Fig. 362), or on a ring of glass or earthenware laid on the top of any suitable vessel. A filter of this kind that will hold one fluid ounce will filter many ounces of some liquids in an hour.