or to be assisted by adding some other body. For
example, camphor powders more easily when a few drops of spirits of
wine are added to it; mace, nutmegs, and such oily aromatic substances
are better for the addition of a little white sugar; resins and
gum-resins should be powdered in a cold place, and if they are
intended to be dissolved, a little fine well-washed white sand mixed
with them assists the process of powdering. Tough roots, like gentian
and calumba, should be cut into thin slices; and fibrous roots, like
ginger, cut slanting, otherwise the powder will be full of small
fibres. Vegetable matter, such as peppermint, loosestrife, senna, &c.,
requires to be dried before it is powdered.

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