The substances extracted by water from vegetables by infusion are chiefly gum, mucus extractive, tannin, certain vegetable acids, the bitter and narcotic principles, gum-resin, essential oil, and alkaloids. Some of these substances are only sparingly soluble in water at ordinary temperatures; but more readily so in hot water, and freely soluble in boiling water. The temperature of the water should be therefore proportioned to the nature of the vegetable matter operated on. For mere ‘demulcent infusions,’ in which starch and gum are the chief substances sought to be dissolved out, and when the active principle is scarcely soluble in water, unless at nearly the boiling temperature, boiling water alone should be employed; but when the medicinal virtues of vegetables are soluble in water at lower temperatures, it is better to employ hot water (165° to 175° Fahr.), and to allow a little longer period for the digestion. In many cases temperate water (from 60° to 70° Fahr.), or tepid water (from 80° to 90° Fahr.), may be used with advantage, especially in the preparation of ‘aromatic bitter infusions,’ and in most cases where it is wished that the product should contain as little inert matter as possible; but when water at low temperatures is
employed, the period of the maceration must be proportionately increased. By adopting the method of maceration in vacuo, or in an atmosphere of carbonic acid, the menstruum may be allowed to lay in contact with the vegetable matter for an unlimited period, without decomposition taking place.
Infusions, like decoctions, are liable to undergo spontaneous decomposition by keeping, especially in warm weather, when a few hours are often sufficient for their passage into a state of active fermentation; they should, therefore, when possible, be prepared for use daily, as beyond twenty-four hours they cannot be depended on. The London College directs a pint only to be made at a time, thus very properly regarding them as extemporaneous preparations.
Concentrated infusions, now so common in the shops, and, unfortunately, so generally used in dispensing, are either made by taking 8 times the quantity of the ingredients ordered in the pharmacopœia, and then proceeding in the usual manner, or by the method of displacement; or, by carefully and rapidly concentrating the simple infusions, by evaporation in a steam or salt-water bath, until reduced to about 1-7th of the original quantity. In either case the liquid is put into a strong bottle, without being filtered, and 10 to 12% of rectified spirit added to it, whilst still hot. The cork is then put in and secured down, and the whole agitated for some minutes, after which it is set aside for a week, when the clear portion is carefully decanted from the sediment for sale. Another method, which answers well with the aromatic bitter vegetables, is to take 8 times the usual quantity of the ingredients, and to exhaust them with a mixture of rectified spirits, 1 part, and distilled water, 3 parts; by digestion, or, better still, by percolation. Concentrated infusions made in this way keep well, and deposit scarcely any sediment. Many houses that are remarkable for the ‘brilliancy’ and beauty of these preparations, employ 1⁄3 spirit of wine and 2⁄3 water as the menstruum. It may, however, be taken, as a general rule, that for vegetable substances that abound in woody fibre, and contain little extractive matter soluble in water (as quassia, for instance), 1⁄6 to 1⁄5 part of spirit is sufficient for their preservation; whilst for those abounding in mucilage or fecula, or that readily soften and become pulpy and glutinous in weak spirit (as rhubarb), 1⁄5 to 1⁄3 is required.
By adopting the method originally suggested by Mr Alsop, infusions may be preserved, uninjured, for a year or longer, without the addition of spirit or any other substance. The only precaution necessary is to keep them in bottles, perfectly filled and hermetically sealed.[360] Our own plan is to put a few bruised cloves or seeds of black mustard into the bottles, which must be only 2-3rds filled, then completely fill them with a condensed atmosphere of carbonic acid gas; and, lastly, to stopper them and seal them over, so as to perfectly exclude the air. A pint of decoction of sarsaparilla and 1⁄2 pint of infusion of calumba, treated in this way, kept good for fully 9 years. By simply macerating in the infusion as much bruised mustard seed as can be added without flavouring the liquor, along with a little bruised cloves, we find that most vegetable infusions may be preserved in bottles which are occasionally uncorked, without either fermenting or becoming mouldy, by the use of very little spirit (1⁄9 or 1⁄10).
[360] ‘Pharm. Journ.,’ i, 57.
Before adding the spirit to infusions made with cold water, or with water which is only tepid, it is advisable to heat the liquid to about 185° Fahr., in a water bath, and after keeping it at that temperature for a few minutes, and allowing it again to become cold, to separate it from the precipitated matter, either by filtration or decantation.
It is often very difficult to render vegetable infusions and decoctions perfectly transparent, a quality always expected in the concentrated preparations. Defecation by repose is always better than filtration, owing to the more or less viscidity of the suspended matter. When this is not sufficient, they may be clarified with white of egg (2 or 3 to the gall.), previously beaten up with 5 or 6 fl. oz. of water. Most of the vegetable infusions and decoctions will readily pass the filter, after a very small quantity of acetic, nitric, or sulphuric acid has been added to them. The most obstinate may be rendered ‘brilliant,’ or ‘candle bright,’ as the ‘cellarmen’ call it, by shaking them up, first with about a drachm of dilute sulphuric acid, and afterwards with the whites of 3 or 4 eggs, previously mixed with a few ounces of water, for each gallon of the liquid. This plan is, however, objectionable for many medicinal preparations.
As many infusions which are occasionally employed in medicine must necessarily escape being separately noticed in this work, it may be as well to remark that the infusions of all vegetables that do not exert a very powerful action on the human frame as ordinary herbs and roots may be made by pouring 1 pint of boiling water on 1 oz. of the vegetable matter, and allowing it to macerate for 1⁄2 an hour to an hour. The decoctions of the same vegetables may be made by simply boiling the above ingredients, in the same proportions, for 10 or 15 minutes, instead of operating by mere infusion. With substances of somewhat greater activity, only half the above quantity should be taken; whilst, with the narcotic plants and those possessing great activity, 1 to 2 dr. to water, 1 pint, will be the proper quantity. The ordinary dose of such infusions and decoctions is 1⁄2 to 1 wine-glassful (1 to 2 fl. oz.), two, three, or four times a day, as the case may indicate.
Infusion is preferred for all bodies of a delicate texture,