Extracts are preparations of vegetable juices obtained by expression, decoction, or infusion, and evaporated down to a solid or semi-solid consistence. They are distinguished, according to their solvents, as aqueous or watery, alcoholic, spirituous (proof or u.p. spirit), acetic (dilute acidulated water), and ethereal. Fluid extracts are those evaporated only to a thin syrupy consistence, and mixed with } to 1/10 volume of rectified spirit. The terms simple and compound distinguish whether one or more substance has been extracted.

The process of preparing pharmaceutical extracts divides itself into two operations - obtaining a solution of the principle required, and evaporating that solution to a dense consistence. The first step is to reduce the solid substance to a state that will admit of- its complete exhaustion by the solvent. This exhaustion is effected by digestion, displacement, decoction, or expression, and the resulting solution is carefully filtered.

The elimination of the excess water, in order to bring the solution to the desired consistence, is usually performed by evaporation. This may be conducted in an open shallow pan, in a water-bath, in a double-jacketed pan heated by steam, or in a vacuum-pan. The first method is objectionable from the danger of incineration; the second is good if 1/5 part of salt be added to the water in the bath, raising its boiling-point nearly 7° F., and thus ensuring an internal temperature of fully 212° F. (100° C). Steam-jacketed pans are commonly used on the large scale; extracts prepared in vacuo are found to be much superior to the ordinary articles.

For several reasons, all these processes would seem to be inferior to that introduced by Prof. Herrara, whose observations satisfied him that, when the water partially congeals, the dissolved principles remain in solution in the mother liquors, and that 2 or 3 congelations are generally sufficient for obtaining the solutions concentrated enough to finish the extract by exposure upon plates to the heat of the sun, or in a drying-closet heated to 86° F. (30° C). Extracts prepared by this method accurately represent the properties of the plants, and those principles which are changed or volatilized by the influence of heat remain unaffected. The apparatus required is very simple, being mainly a modification of the sorbetiere or ice-cream freezer. The freezing mixture may be ice and salt, or ice and calcium chloride. As the congelation progresses, the ice-cake is removed, broken up, and pressed, to separate the mother-liquor as completely as possible, which is finished by evaporation in shallow dishes.

Extracts should be preserved out of contact with the air as soon as they are prepared. When in pots, the inner surface of the bladder used to tie them down should be moistened with a few drops of oil of cloves or creosote. Hard extracts may be kept in gut-bladders, covered over in stone pots. The essential qualities of a good extract are: - (1) Freedom from grit, and complete solubility in 30 parts of the solvent used in its preparation, forming an almost clear solution; (2) proper consistence, and uniform colour and texture. Extracts should be rejected as worthless when over 6 months old. The following are some of the chief kinds.

Aconite

(1) Bruise 112 lb. fresh leaves and flowering tops, press out the juice, heat it gradually to 130°; F. (54 1/2° C), and separate the green matter by a calico filter. Heat the strained liquor to 200° F. (93 1/2° C), to coagulate albumen, and again filter. Evaporate the filtrate by a water bath to the consistence of a thin syrup; add the green colouring matter previously separated, and, stirring the whole together assiduously, evaporate at a temperature not exceeding 140° F. (60° C.) to a pill consistence.

(2) Bruise in a mortar 1 lb. fresh leaves of aconite, express the juice, and evaporate it, unstrained, to a proper consistence.

(3) Beat the fresh leaves of aconite to a pulp, and express the juice; subject the residue to percolation with rectified spirit until the latter passes through without being materially coloured; unite the expressed juice and the percolated tincture, filter, distil off the spirit, and evaporate in a vapour or a water bath to a proper consistence.

(4) Alcoholic

1 lb. aconite in coarse powder, 2 1/2 pints proof spirit; proceed by displacement, and when all the spirit has penetrated the powdered mass, keep this covered with distilled water, until the liquid begins to cause a precipitate in falling into that which has previously passed through; next distil the spirit from the tincture, and evaporate the residue to the proper consistence.

(5) 1 lb. aconite; 1 qt. or q.s. spirit, sp. gr. .935 (=13 u.p.); as last.

(6) From the tincture prepared with rectified spirit, and by either maceration or displacement.

(7) The juice is expressed from the fresh herb, which is then sprinkled with about 1/3 of its weight of water, and again pressed; the mixed and strained liquid is evaporated in a vapour-bath at 122° to 140° F. (50°-60° C), to about 1/2; to this, as soon as cold, an equal weight of spirit (sp. gr. '900) is added; and after frequent agitation for 24 hours, the whole is filtered, with pressure; the marc is treated with fresh spirit (equal to about J that first used) and again pressed; the mixed liquors are filtered and evaporated, as before, to the proper consistence.

(8) Ammoniated

1 dr. extract of aconite, 10 or 12 drops strongest liquor of ammonia; mix.

(9) Dried

The expressed juice, strained through a sieve or coarse linen, is at once exposed in earthen dishes, in layers of about 2 lines deep, in a stove or current of dry air, to a temperature ranging between 95° and 104° F. (35°-40° C), until reduced to dryness. The dried extract is packed in bottles.

(10) Boot