It has a strong taste, varying in different plants; it is soluble in water, and in alcohol when it contains water, but is quite insoluble in absolute alcohol and æther; its aqueous solution soon runs into a state of putrefaction; by repeated solutions and evaporations, or by long ebullition, it acquires a deeper colour, and in consequence of its combination with oxygen it becomes insoluble and inert, a fact which is of extreme importance as it regards its pharmaceutical relations; it unites with alumine, and if boiled with its salts, precipitates it, hence wool, cotton, or thread, impregnated with alum, may be dyed of a fawn-colour by extractive; its habitudes with alkalies are very striking, combining most readily and forming with them compounds of a brownish yellow colour, which are very soluble in water; if to a colourless and extremely dilute solution of extractive, an alkali be added, a brown or yellowish tint is immediately produced, so that under certain circumstances I have found an alkali to be a serviceable test in detecting the presence of extractive matter. The usual brown hue of the liquor ammoniæ acetatis, is owing to the action of the ammonia upon traces of vegetable extractive contained in the distilled vinegar.
Much confusion has arisen from the word extract having been employed in this double meaning,—chemically to express a peculiar vegetable proximate principle, and pharmaceutically to denote any substance however complicated in its nature, which has been obtained by the evaporation of a vegetable solution or a native vegetable juice. It is in the latter sense that it is to be understood in the present article.
The different proximate principles of vegetable matter undergo various and indefinite changes with such rapidity, when acted upon by heat, that the process of extraction must necessarily more or less impair the medicinal efficacy of a plant, and not unfrequently destroy it altogether, and hence, says Dr. Murray, “with the exception of some of the pure bitters, as gentian, or some of the saccharine vegetables, as liquorice, there is no medicine perhaps but what may be given with more advantage under some other form;” this however is not exactly true, for when care is taken in the preparation, we are thus enabled to concentrate many very powerful qualities in a small space, and the process lately adopted of evaporating the solutions by the aid of steam, contributes very materially to obviate the failures which so frequently occurred from a too exalted temperature. There is, for instance, great reason to suppose that the black colour which so often characterises the extracts of commerce, is frequently owing to the decomposition and carbonization of the vegetable matter; the colour therefore of an extract becomes in some degree a test of its goodness. I have lately examined the extracts of commerce with some attention, and I find the presence of iron by no means an uncommon circumstance; when thus contaminated they afford a very dirty coloured solution, which rapidly becomes darker on exposure to air. The extracts mentioned in the preface as made by Mr. Barry, by evaporating in vacuo, deserve the attention of the profession; the principle is without doubt well calculated to secure the active matter of the plant from those changes to which it is constantly liable during the ordinary operation of inspissation. The extracts, thus prepared, are certainly more powerful in their effects, and some few of them appear also to possess properties which are not to be distinguished in the Extracts of Commerce; those of narcotic plants, as Hemlock, Hyoscyamus, &c. are decidedly more efficacious; where the practitioner directs their use, he should, to prevent any mistake, add the words in vacuo præp. as in Formula 4; for on account of the difference in the strength of these preparations, and of those prepared by the ordinary method, they cannot be indiscriminately employed. Dr. John Davy, at my request, has made trial of these extracts in the Military Hospital at Fort Pitt, and as his results coincide with those obtained in my own practice, I shall relate, under the history of each Extract, the comparative conclusions which have been obtained.
1. Watery or Simple Extracts.
Mucilaginous Extracts of Rouelle.
These extracts must, of course, contain all the principles of a plant which are soluble in water, such as gum, extractive matter, tannin, cinchonin, sugar, fecula, &c. together with any soluble salts which the vegetable may contain. I have also found by experiment that an aqueous extract may even contain, in small proportions, certain elements which, although quite insoluble in water, are nevertheless partially soluble in vegetable infusion. This law of vegetable chemistry has never been expressed, although we have repeated instances of its truth, and a knowledge of it may explain some hitherto unintelligible anomalies. It has been stated that extractive matter is perfectly insoluble in æther, but Mr. A. Thomson found repeatedly, that if a small portion of resin was present, æther would in that case take up extractive in combination with the resin which it so readily dissolves. As decoction or infusion is a process preliminary to that of extraction, the practitioner must refer to those articles for an enumeration of the different sources of error which are attached to them.
Extractum Aloes Purificatum. L. The resinous element of the aloes is got rid of in this preparation; on which account it is supposed, in an equal dose, to be more purgative and less irritating. Dose, gr. x to xv. Form: 12, 13, 36.
Extractum Anthemidis. L.E. Extract. Florum Chamœmeli. D. This extract furnishes an example of the change effected on some plants by the process of extraction; in this case the volatile oil is dissipated, and a simple bitter remains, possessing scarcely any of the characteristic properties of chamomile. This remark, however, does not apply to this extract, when prepared in vacuo. I have lately received from Mr. Pope of Oxford Street, a specimen which retains, in the most eminent degree, all the odour and taste of the recent flower. Dose, gr. x–℈j.
Extractum Cinchonæ. L.D. The properties of the bark in this preparation are much invalidated, owing to the oxidizement of its extractive matter, which takes place to such an extent, that not more than one half of the preparation is soluble in water; it is not however altogether devoid of utility, and will often sit lightly on the stomach, when the powder is rejected. Its taste is very bitter, but less austere than the powder. The most beautiful extract of bark, which I have ever seen, was prepared by Mr. Barry of Plough Court; its colour was that of a deep brilliant ruby, and its flavour preserved all the characteristic peculiarity of the recent substance.[[499]] Dose, grs. x to ʒss. Fourteen ounces of the bark will yield about three ounces and a half of extract. It should be kept soft, so as to be fit for forming pills, and hard, so that it may be reduced to powder.
Extractum Colocynthidis. L. This extract is much milder, although less powerful, than the pulp; Dose, grs. v to ʒss. It soon becomes hard and mouldy.[[500]]