5 ( J1 Preparations and Compost lions. Part III.

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Water is the menstruum most economically employed inmaking extracts, as it is capable of dissolving all the activeprinciples except resin, and can have its solvent powers assist-ed by a considerable degree of heat.

Watery extracts are prepared by boiling the subject in wa-ter, and evaporating the strained decoction to a thick consis-tence.

It is indifferent, with regard to the medicine, whether thesubject be used fresh or dry ; since nothing that can be pre-served in this process will be lost by drying. With regardto the facility of extraction, however, there is a very consider-able difference ; vegetables in general giving out their virtuesmore readily when dried than when fresh.

In many cases, it is necessaiy to assist the action of themenstruum by mechanical division, but it should not be car-ried so far as to reduce the substance to a very fine powder;as Fabbroni found that cinchona, at least, yielded a largerproportion of extract, when only coarsely powdered.

The quantity of water ought to be no greater than is ne-cessary for extracting the virtues of the subject This point,however, is not very easily ascertained ; for, although someof the common principles of extracts be soluble in a verysmall proportion of water, there are others, such as the tan-nin, of which water can dissolve only a certain proportion,and cannot be made to take up more by any length of boil-ing ; besides, we have no very good method of knowingwhen we have used a sufficient quantity of water ; for vegeta-ble substances will continue to colour deeply successive por-tions of water boiled with them, long after they are yieldingnothing to it but colouring; matter. One of the best methodsis to boil the subject in successive quantities of water, as longas the decoctions form a considerable precipitate with the testwhich is proper for detecting the substance we are extracting,such as a solution of gelatin for tannin, of alum for extrac-tive, &c.

The decoctions are to be evaporated after they have beenfiltered boiling hot, without any farther depuration; becausesome of the most active principles of vegetable substances,such as tannin, are much more soluble in boiling than in coldwater, and because almost ail of them are very qurckly affect-ed by exposure to the atmosphere. Therefore, if a boilingdecoction, saturated with tannin, be allowed to cool, thegreatest part of the very princip e on which, the activity of thesubstance depends, will separate to the bottom, and, accord-ing to the usual directions, will be thrown away as sediment.