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668. Care of the Mortar. Be careful not to pound too hard in glass, porcelain, or Wedgwood-ware mortar; they are intended only for substances that pulverize easily, and for the purpose of mixing or incorporating medicines. Never use acids in a marble mortar, and be sure that you do not powder galls or any other astringent substances in any but a brass mortar. 669. Sifting. Sifting is frequently required for powdered substances, and this is usually done by employing a fine sieve, or tying the powder up in a piece of muslin, and striking it against the left hand over a piece of paper. 670. Filtering. Filtering is frequently required for the purpose of obtaining clear fluids, such as infusions, eye-washes, and other medicines; and it is, therefore, highly important to know how to perform this simple operation. First of all take a square piece of white blotting paper, and double it over so as to form an angular cup. Open out this filter paper very carefully, and having placed it in a funnel, moisten it with a little water. Then place the funnel in the neck of the bottle, and pour the liquid gently down the side of the paper, otherwise the fluid is apt to burst the paper. 671. Maceration. Maceration is another process that is frequently required to be performed in making up medicines, and consists simply in immersing the medicines in cold water or spirits for a certain time. 672. Digestion. Digestion resembles maceration, except that the process is assisted by a gentle heat. The ingredients are placed in a flask, such as salad oil is sold in, which should be fitted with a plug of tow or wood, and have a piece of wire twisted round the neck. The flask is held by means of the wire over the flame of a spirit lamp, or else placed in some sand warmed in an old iron saucepan over the fire, care being taken not to place more of the flask below the sand than the portion occupied by the ingredients.