These are preparations obtained by evaporating aqueous and alcoholic solutions of vegetable substances, until a mass of a somewhat firm, tenacious consistence remains. When water has been employed for making the solution, the extract may consist of gum or mucilage, albumen, extractive and saccharine matter, in conjunction with the active principles, and other salts which the vegetable contained, and is termed a Watery Extract; but if alcohol have been the menstruum, resin, extractive, and all the above matters, except gum, may be the ingredients, and the extract is denominated a Spirituous Extract, The latter appellation also is used if proof spirit be employed. The proper menstruum, therefore, for the preparation of any extract, must be that fluid which most readily dissolves the peculiar principles on which the medicinal efficacy of the vegetable is supposed to depend.

When water is employed, the substance to be subjected to its action should be in the dried state, and coarsely powdered; and the solution, whether made by decoction or infusion, should be evaporated immediately after it is strained, and whilst it is yet hot; for, as we observed in treating of decoctions, water at the temperature of 212° takes up much more of the active matter of vegetables than it can hold in solution at a lower temperature; therefore by allowing them to cool, with the view of defecation, and evaporating the clear fluid only, a considerable portion of the active matter does not enter into the extract, and is necessarily lost. In performing the evaporation, a higher temperature than that of boiling water must not be employed; but it must, nevertheless, be conducted as quickly as possible; and, therefore, the evaporating vessel should be broad and shallow, and set in boiling water; or the water bath recommended by Dr. Powell2 should be employed. (See Instruments, Part i.) The method of preparing extracts in vacuo introduced by Mr. Barry, is a great improvement; and certainly, if the presence of air is likely to alter the properties of extracts, considerable advantages will accrue from Mr. Barry's mode of conducting the evaporation.3

1 Chemists are much divided as to the nature of the substances to which the term Extract can be chemically applied; the term, therefore, as used in this work and in the pharmacopoeias, is confined solely to preparations obtained in the manner described under the title, without reference to their chemical properties,

2 Translation of the London Pharmacopoeia, p. 201.

3 For a description of the apparatus Mr. Barry employs, see Journal of Science and the Arts, vol. viii. p. 360.

Alcohol is used only in cases where the active ingredient of the vegetable is chiefly resin, or where it is too volatile to bear the heat which is necessary for evaporating the water without being dissipated, or without suffering some decomposition, which would materially alter its properties. A tincture of the substance is first obtained, which is then evaporated by a very gentle heat in a water bath; but the alcohol need not be allowed to evaporate in the air, as by employing a distilling apparatus the greater part of it is again obtained, either altogether free from any vegetable matter, or containing a small portion only of the more volatile principle; which renders it fitter for being again employed for the preparation of the same kind of extract.

Whether water, proof spirit, or pure alcohol be employed, the medicinal properties of the extract are always in some degree injured; the volatile parts are dissipated, and some of the fixed parts decomposed by the degree of heat required for the evaporation,-particularly if water be the menstruum; or the proper extractive is oxidized, and consequently rendered inert. Such are some of the objections to these preparations, as they are usually found in the shops; but well prepared, they are excellent medicines. As a general rule, they should be made by expressing the juice from the recently gathered vegetable, just getting into full flower, and inspissating this expressed juice as rapidly as possible by exposing it in thin strata to a current of very dry air. Practical experiments have fully demonstrated the advantage this process posseses over all others at present in use; for it was shown that 10 grains of conium extract, thus prepared, were more than equal to 20 grains of that prepared in vacuo, and more than equal to 60 grains of that prepared by the process of boiling down the juice to an extract.1

Extracts require to be kept in a hard and a soft state. A hard extract should be in such a state as to admit of its being easily pulverized, and the soft extract should be such as to retain the round form of a pill, without the addition of any powder. Both kinds should be preserved in pots which are so close that all the external air can be excluded; and these should be kept in a dry place. The soft should be wrapped in oiled bladder, and kept also in close covered pots.

The London College does not arrange the extracts under the titles, Watery and Spirituous, which is the arrangement of the Edinburgh College; nor does it distinguish them by the terms Simple and Resinous, in the manner of the Dublin

1The best extracts which I have seen are prepared in this manner by Mr. Squires of Oxford Street, the Chemist to the Queen.

Pharmacopoeia. It classes the inspissated juices, which require no menstrua as extracts. The following general directions are given by the London College for the preparation of extracts: -

"In preparing all kinds of extracts, unless otherwise directed, evaporate the fluid as quickly as possible in a broad, shallow dish placed in a water bath, until the extract acquire a consistence proper for forming pills; and, towards the end of the operation, stir assiduously with a spatula.

" Sprinkle a small quantity of rectified spirit upon all the softer extracts, to prevent them from becoming mouldy."

The Edinburgh College gives its general directions for the preparation of the extracts by water, under the extract of gentian; and for the extracts by water and alcohol, under the extract of bark.