All vegetable substances twist be pre viously dried. Resins and gummy resins, which become soft in summer, must be powdered in very cold weather, and must be beaten gently, or they will be convert ed into a paste instead of being powder ed. Wood, roots, barks, horn, bone, ivory, &c. must be previously cut, split, chipped, or rasped. Fibrous woods and roots should be finely shaved after their bark is removed, for otherwise their powders will be full of hair-like filaments, which can scarcely be separated. Some sub stances will even require to be moistened with mucilage of tragacanth, or of starch, and then dried before they can be pow dered. Camphor may be conveniently powdered by the addition of a little spirit of wine, or almond oil. The emulsive seeds cannot be reduced .to powder un less some dry powder be added to them. -To aromatic oily substances sugar is the best addition. All impurities and inert parts having been previously separated, the operation must be continued and re peated upon vegetable substances till no residuum is left. The powders obtained at different times must then be intimately mixed together, so as to .bring the whole to a state of perfect uniformity.
Very hard stony substances must be re peatedly heated to a red heat, and then suddenly quenched in cold water, until they become sufficiently friable. • Some metals may be powdered hot in a heated iron mortar, or may be rendered brittle by alloying them with a little mercury.
Trituration is intended for the still more minute division of bodies. It is perform ed in flat mortars of glass, agate, or other hard materials, by giving a rotatory mo tion to the pestle; or on a tevigating stone, which is generally of porphyry, by means of a muller of the same substance. On large quantities it is performed by rollers of hard stone, turning horizontally upon each other, or by one vertical roller turning on a flat stone.
The substances subjected to this ope ration are generally previously powdered or ground.
Levigation differs from trituration only in the addition of water or spirit of wine to the powder operated upon, so as to form the whole mass into a kind of paste, which is rubbed until it be of sufficient smoothness or fineness. Earths and some metallic substances are levigated.
Granulation is employed for the me chanical division of some metals. It is performed, either by stirring the melted metal with an iron rod until it cools, or by pouring it into water, and stirring it continually as before, or by pouring it into a covered box, previously well rubbed with chalk, and shaking it until the metal cools, when the rolling motion will be converted into a rattling one. The ad hering chalk is then to be washed away.
Mechanical Separation is obtained by sifting, elittriation, decantation, filtration, despumation, expression.
Sifting. From dry substances, which are reduced to the due degree of minute ness, the coarser particles are to be sepa rated by sieves of iron-ware, hair-cloth, or gauze, or by being dusted through bags of fine linen. For very light and va
luable powders, or acrid substances, com pound sieves, having a close lid and receiver, must be used. The particles which are not of sufficient fineness to pass through the interstices of the sieve, may be again powdered.
Elutriation is confined to mineral sub stances, on which water has no action. It is performed by separating them from foreign particles and impurities, of a dif ferent specific gravity, in which case they are said to be washed ; or for separating the impalpable powders, obtained by tri. turation and levigation from the coarser particles. This process depends upon the property that very fine or light pow ders have of remaining for some time suspended in water ; and is performed by diffusing the powder or paste formed by levigation through plenty of water, let ting it stand a sufficient time, until the coarser particles settle at the bottom ; and then pouring off the liquid in which the finer or lighter particles are suspend ed. Fresh water may be poured on the residuum, and the operation repeated ; or the coarser particles, which fall to the bottom, may be previously levigated a se cond time.
Decantation. The fine powder which is washed over with the water is separated from it, by allowing it to subside com pletely, and by either decanting off the water very carefully, or by drawing it off by a syringe or syphon. These processes are very frequently made use of for sepa rating fluids f'rom solids which are speci fically heavier, especially when the quan tity is very large, or the solid so subtle as to pass through the pores of most sub stances employed for filtration, or the liquid so acrid as to corrode them Filtration. For the same purpose of se parating fluids from solids, straining and filtration are often used. These differ only in degree, and are employed when the powder either does not subside at all, or too slowly and imperfectly for decanta tion. The instruments for this purpose are of various materials, and must in no instance be acted upon by the substances for which they are employed. Fats, re sins, wax, and oils, are strained through hemp or flax, spread evenly over a piece of wire-cloth or net stretched in a frame. For saccharine and mucilaginous liquors, fine flannel may be used : for some saline solutions, linen. Where these are not fine enough, unsized paper is employed ; but it is extremely apt to burst by hot watery liquors, which dissolve its size ; and very acrid liquors, such as acids, are filtered by means of a glass-funnel, filed with pow dered quartz, a few of the larger pieces be ing put into the neck, smaller pieces over these, and the finer powder placed over all. The porosity of this last filter retains much of the liquor ; but it may be reco vered by gently pouring on it as much distilled water ; the liquor will then pass through, and the water be retained in its place.