bag of lawn is dropped into the canister, the top being turned over the edge; the powder to be sifted is put in the bag, the lid put on, and, by tapping and shaking, the finest portions pass into the canister without any escaping into the air—a point of very considerable importance where the powder is irritating or expensive. Various contrivances are constantly had recourse to, in order to render certain substances more readily pulverisable, the contrivance varying very much with the peculiarities of the substance. We will mention a few of these, as they may afford useful suggestions in cases of difficulty. All vegetable, and many mineral substances, are much more readily powdered after having been _thoroughly_ dried; so far is this process carried, that many drugs are dried so as to lose fifteen per cent. of their weight before powdering. In proof of the utility of the drying, let any person try to powder a piece of whiting as it comes from the oilman’s; it will be found to cake together, and be more readily powdered; if dry, however, it powders with the greatest ease. After drying, substances should not be exposed to the air; but, unless they are of such a nature as to be softened by heat, are better operated on while still warm. Flints are more readily powdered by being heated to redness and quenched in cold water; charcoal, for tooth-powder, while still warm from drying. Gum can only be powdered whilst perfectly dry. Camphor, which is with great difficulty powdered alone, yields readily if a drop or two of spirit is poured on it. Substances which clog together and cake under the pestle, are not uncommon; to these it is sometimes requisite to add sand, which may afterwards be separated—this prevents the clogging; but its use is often impracticable. Lime, if required in very fine powder, for dusting over plants to kill slugs, &c., is readily obtained by slaking it, when fresh burned, with _boiling_ water; when, if too much water is not used, it falls into an exceedingly fine powder. Sal-ammoniac, and some other saline bodies, are most readily powdered by dissolving them in as small a quantity of boiling water as possible, and stirring the solution rapidly as the water is boiled away, or as the solution cools. Before dismissing the pestle and mortar, we may allude to their use in mixing powders together, although a much more ready mode of doing this is with a sieve. Two or more powders stirred together, and passed two or three times through a sieve, are much more intimately mixed, than if rubbed for a long time in a mortar. Metals cannot be divided in the mortar; the most convenient mode of proceeding, if they are fusible under a white heat, is to melt them, and pour them whilst liquid into a pail of water, which should be full to avoid any spluttering, and the hotter the metal, the more filmy the particles. It is scarcely requisite to state, that the metal should be poured in a circle, so as not to collect at one place.