748

Decoctions.

part in.

volatile matters, or by favouring the oxidizement of the ex-tractive, which, in a continued temperature of 212°, attractsthe oxygen of the atmosphere so rapidly, that it is soon con-verted into a solub/e, insipid, inert matter, and precipitated inthe fluid. This is the case with some substances, which arenevertheless ordered to be prepared in this form by the col-leges, and which we shall particularly notice in treating of theindividual decoctions.

For making decoctions, the substances employed must bedivided, if in the dry state, by pulverization, or, if fresh, byslicing, so as to expose an extended surface to the action ofthe water; which is thus enabled to take up their soluble prin-ciples in a shorter space of time, a circumstance, for the reasonsalready stated, of much importance in the preparation of de-coctions. By covering the vessel in whicli they are made, theaction of the air is prevented from affecting the ingredients;but there is reason for believing, that by long coction in water,even in covered versels, the constituents of some vegetable bodiesre-act upon one another, and produce entirely new com-pounds, possessed of properties altogether different from thosewhich they previously constituted. On this account, decoc-tions should be quickly made; and when aromatic or volatileingredients are to enter into them, these should not be boiledwith the more fixed substances, but the decoction, after itis made, should be poured over them, and allowed to remaincovered up until it is nearly cold, before it be strained. Ingeneral, however, it is better to strain decoctions while theyare hot through a sieve: for as boiling water dissolves a largerproportion of vegetable matter than it can retain in solutionat a lower temperature, a deposit almost always takes place asthe decoction cools; and if this be of active matter, it is lostby deferring the straining; whereas by straining the decoctionwhile hot, the deposit can be mingled, by being shaken, withthe clear fluid, when it enters into extemporaneous composi-tions, or when the dose of it is taken.

Decoctions, from the nature of their constituents, very soonferment and spoil: consequently, they should be prepared insmall quantities only, and never used, particularly in summer,forty-eight hours after they have been made.

DECOCTUM ALOES COMPOSITUM, Lond. Com-pound Decoction cf Aloes.

" Take of extract of Liquorice, half un ounce ,• Subcarbonateof Potass, two scruples ; extract of spiked Aloes, powdered,Myrrh, powdered, Saffron, of each, a drachm ; water, a pint.

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