Sect. %

Pharmaceutical Operations. Ixxxvii

ces for which they are employed. Fats, resins, wax, andoils, are strained through hemp or flax, spread evenly over apiece of wire cloth or net stretched in a frame. For saccha-rine and mucilaginous liquors, fine flannel may be used; forsoirie saline solutions, linen. Where these are not fine enough,unsized paper is employed, but it is extremely apt to burst byhot watery liquors. Very acrid liquors, "such as acids, arefiltered by means of a glass funnel, filled with powderedquartz, a few of the larger pieces being put in the neck,smaller pices over these, and the fine powder placed over all.The porosity of this last filter retains much of the liquor; butit may be obtained by gently pouring on it an equal quantityof distilled water ; the liquor will then pass through, and thewater will be retained in its place.

396. Water may he filtrated in large quantises throughbasins of porous stone, cr artificial basins of nearly equal pansof fine clay and coarse sand. In large quantities it may beeasily purified per ascensum, the purified liquor and impuri-ties thus taking opposite directions. The simplest apparatusof this kind is a barrel, divided perpendicularly, by a boardperforated with a row of holes along the lower edge. Intoeach side, as much well washed sand is put as will cover theseholes an inch or two, over which must be placed a layer ofpebbles to keep it steady. The apparatus is now fit for use.W'ater pouted into the one half will sink through the sand inthat side, pass through the holes in the division to the other,and rise through the sand in the other half, from which itmay be drawn by a stop-cock.

397. The size of the filters depends on the quantity of mat-ter to be strained. When large, the flannel or linen is formedinto a conical bag, and suspended from a hoop or frame; thepaper is either spread on the inside of these bags, or foldedinto a conical form, and suspended by a funnel. It is ot* ad-vantage to introduce glass rods or quills between the paperand funnel, to prevent them from adhering too closely.

398. What passes first is seldom fine enough, and must bepoured back again, until by the swelling of the fibres of thefilter, or filling up of its pores, the fluid acquires the requisitedegree of limpidity. The filter is sometimes covered withcharcoal powder, which is a useful addition to muddy anddeep-coloured liquors. The filtration of some viscid substan-ces is much assisted by heat.

399. Expression is a species of filtration, assisted by me-chanical force. It is principally employed to obtain the juicesof fresh vegetables, and the unctuous vegetable oils. It is per-