Chap. XXXV. Of Extracts and Resins.

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The same objection applies more strongly to allowing the de-coction to cool, and deposite a fresh sediment, after it hasbeen partially evaporated. Besides, by allowing the decoc-tions to stand several days before we proceed to their evapo-ration, we are, in fact, allowing the active principles contain-ed in the decoction to be altered by the action of the air, andto be converted into substances, perhaps inactive, which alsoare thrown away as sediment.

The evaporation is most conveniently performed in broadshallow vessels ; the larger the surface of the liquor, the soon-er will the aqueous parts exhale. This effect may likewise bepromoted by agitation.

When the matter begins to grow thick, great care is ne-cessary to prevent its burning. This accident, almost una-voidable it' the quantity be large, and the fire applied, asusual, under the evaporating basin, may be effectually pre-vented, by pouring the extract, when it has acquired the con-sistence of a syrup, into shallow tin or earthen p;ms, and pla-cing these in an oven with its door open, moderately heated;which, acting uniformly on every part of the liquid, will soonreduce it to any degree of consistence required. This maylikewise be done, and more securely, by setting the evapora-ting vessel in boiling water; but the evaporation is in this■way very tedious. Dr Powell has figured a modifkation ofthe common tin sauce-pan for this purpose. It is nothingbut putting a tin evaporating dish over a sauce-pan filled withwater, which is made to boil.

Alcohol is much too expensive to be employed as a men-struum for obtaining extracts, except in those cases wherewater is totally inadequate to the purpose. These cases are,

1st, When the nature of the extract is very perishablewhen dissolved in water, so that it is liable to be decomposedbefore the evaporation can be compleud, especially if we can-not proceed immediately to the evaporation.

c 2rliy, When water is totally incapable of dissolving the sub-stance to be extracted ; and,

3dtt/, When the substance extracted can bear the heat (ofboiling alcohol without being evaporated, but would be dissi-pated by that of boiling water; that is, when it requires aneat greater than 176°, and less than 212°, for its evapori-zation.

In the last case, the alcohol must be perfectly free from wa-ter, because the heat necessary to evaporate it at the end ofthe process would frustrate the whole operation. Hence, al-so, the subject itself ought always to be dry: those substances,

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