Water may be filtered in large quanti ties through basins of porous stone, or artificial basins of nearly equal parts of fine clay and coarse sand. The size of the filters depends on the quantity of matter to be strained. When large, the flannel or linen is formed into a conical bag, and suspended from a hoop or frame; the paper is either spread on the inside of these bags, or folded into a conical form, and suspended by a funnel. It is of ad vantage to introduce glass rods, or quill barrels, between the paper and funnel, to prevent them from adhering too closely. "%What passes first is seldom fine enough, and must be poured back again, until, by the swelling of the fibres of the filter, or filling up of its pores, the fluid acquires the requisite degree of limpidity. The filter is sometimes covered with charcoal powder, which is a useful addition to muddy and deep-coloured liquors. The filtration of some viscid substances is much assisted by heat: Expression is a species of filtration, as sisted by mechanical force. It is princi pally employed to obtain the juices of fresh vegetables, and the unctuous vege table oils. It is performed by means of a screw press with plates of wood, iron, or tin. The subject of the operation is pre viously beaten, ground, or bruised. It is then inclosed in IA bag, which must riot be too much filled, and introduced between the plates of the press. The bags should be of hair.cloth, or canvass inclosed in hair-cloth. Hempen and wooden bags are apt to give vegetable juices a disagreea ble taste. The pressure should be gen. tle at first, and increased gradually. Ve getables intended for this operation should be perfectly fresh and freed from all im purities. In general they should be ex. pressed as soon as they are bruised, for it disposes them to ferment ; but "subacid fruits give a larger quantity of juice and of finer quality, when they are allowed to stand some days in a wooden or earthen vessel after they are bruised. , To some vegetables which are not juicy enough of themselves, the addition of a little water is necessary. Lemons and oranges must be peeled, as their skins contain a great deal of essential oil, which would mix with the juice. The oil itself may be ob taMed separately, by expression with the fingers against a plate of glass.
For unctuous seeds, iron plates are used ; and it is customary not only to heat the plates, but to warm the bruised seeds in a kettle over the fire, after they have been sprinkled with some water, as by these means the product is increased, and the oil obtained is more limpid. But as their disposition to rancidity is in creased by it, if possible, this practice should be laid aside, or confined to ex posing the bruised seeds, inclosed in a bag, to the steam of hot water.
Despumation is generally practised on thick and clammy liquors, which contain much slimy and other impurities, not easily separable by filtration. The scum arises either by simply heating the liquor, or by clarifying it, which is done by mix ing with the liquor, when cold, whites of eggs well beaten with a little water, which on being heated coagulates, and entangling the impurities of the liquor, rises with them to the surfhce, and may be easily removed by a perforated ladle ; or the liquor may now be filtered with ease. Spirituous liquors are clarified by means of isinglass dissolved in water, or any albuminous fluid, such as milk, which coagulates by the action of alcohol with out the assistance of heat. Some ex.
pressed juices, such as those of the an tiscorbetic plants, are instantly clarified by the addition of vegetable acid, such as the juice of bitter oranges.
Fluids can only be separated from each other when they have no tendency to combine, and when they differ in speci fic gravity. The separation may be ef fected by skimming off the lighter fluid 11111th a silver or glass spoon ; or by draw ing it off by a syringe or syphon ; or by means of a glass separatory, which is an instrument having a projecting tube, ter minating in a very slender point, through which the heavier fluid alone is permitted to run ; or by means of the capillary at. traction of a spongy woollen thread ; for no fluid will enter a substance, whose pores are filled by another for which it has no attraction ; and, lastly, upon the same principle, by means of a filter of unsized paper, previously soaked in one of the fluids, which in this way readily passes through it, while the other remains behind. • Mechanical mixture is performed by agitation, trituration, or kneading ; but these will be best considered in treating of the forms in which medicines are ex hibited.
Chemical Operations and Results.
Under this chapter we have to consider the apparatus employed, the changes produced, and the general analyses that ensue.
The apparatus consists of vessels, fuel, or heat ; and the different modes by which such fuel or heat is applied, whe ther lamps, furnaces, &c.
The vessels must necessarily vary in their form and materials; upon the first of which it will be more convenient to enlarge, as we proceed to treat of the particuir operations in which they are employed. In choosing the materials for the construction of our vessels, the properties most generally required are a power of resisting chemical agents, tran sparency, compactness, strength, fixity, and infusibili v, and an ability to sustain sudden variations of temperature without breaking.
Generally speaking, metals possess the four last properties in considerable per fection ; but they are all opaque. Iron and copper are apt to be corroded by chemical agents ; and a solution of the last is often followed by dangerous affee tions. Tinning them will sometimes, but not always, answer ; for tin and lead are often too fusible. Platinum, gold, and silver, resist most of the chemical agents, but are too expensive for general use.
Good earthenware resists the greatest intensity of heat, but has no other pro perty to recommend it. Clay, the basis of all such wares, is plastic when worked with water, sufficiently hard when burnt with an intense heat. But intense heat contracts it unduly, and it is apt to split and crack upon exposure to sudden changes of temperature ; whence it is necessary to counteract this property by the addition of some other substance. Siliceous sand, clay reduced to powder, and then burnt with a very intense heat, and plumbago, are occasionally used. These additions, however, are attended with other inconveniences ; plumbago especially is liable to combustion, and sand diminishes the compactness; so that when not glazed they are porous, and when glazed they are acted upon by chemical agents. The chemical vessels manufactured by Messrs. Wedgewood are the best of this description, except porcelain, Which is too expensive.