The different classifications of plants by Ray, Tournefort, Linnaeus, and other eminent botanists, having already been mentioned, vol. i. pp. 316-17 ; we shall at present confine our attention to the constituent parts of vegetables, and conclude with a few directions for collecting and preserving simples, both for medicinal and economical purposes.
According to the most accurate chemical analysis, plants have been found to contain:
1. Sugar ; to which we refer.
2. Starch ; which see.
3. Gluten is a viscid matter, that remains after washing wheaten flour, in the preparation of starch, and which also subsides when paste is repeatedly washed with cold water, till that fluid become perfectly transparent. It is very tenacious ; and, if combined with the nitric, it maybe converted into the oxalic acid, which is known under the name of Essential Salt of Lemons.
Q. Caoutchouc, or Elastic Resin, is obtained not only from the tree mentioned in that article, but exists in numerous plants, particularly the misletoe, from which it may be extracted by infusion in water.
14. Earths : - for instance, lime, which abounds in almost every plant; silica, or pure flint, has been -discovered in the different species of Horsetail and Grass ; magnesia, which occurs chiefly in the marine plants, particularly the Sea-wrack, and the exotic species of the Saltwort, or Salsola soda; and lastly, alumina, or pure clay, which is found only in a few vegetables.
15. Several metallic substances have likewise been chemically separated ; though in very small portions. Thus M. Sage has clearly proved that gold exists in many plants; M. M. Scheele and Prouse have obtained manganese from the ashes of the Vine, Green Oak, Fig-tree, Pine, etc. Iron also forms an ingredient in almost every plant, but particularly in the different species of the Saltwort.
16. The Woody Fibre is the basis of all vegetables: it abounds mostly in trees, but occurs less frequently in herbs; is insoluble either in water or alkohol, but is act-ed upon by alkalies; and is also decomposed by the mineral acids : when burnt in a smothering heat, it yields a large proportion of Carbon, or Charcoal.
The mode of collecting vegetables, or simples, varies according to their peculiar nature, and the different parts to be preserved. Thus, annual roots should be dug up before they shoot forth either stalks or flowers; biennials in the autumn of the year in which they were sown; and perennials also in the autumn : though some natura-prefer the toting for the collection of the latter ; because at that season they abound with juice, or sap : but, as they are then apt to shrivel in drying, the autumn is doubtless the most proper time for such purpose, herbs ought to be gathered, when the leaves have attained their full growth, though previously to the appearance of the flower-buds. With respect to the removal of leaves, no certain rule can be given ; but flowers should be plucked on a clear day, when they are moderately expanded : after having been carefully selected, both herbs and flowers must be cautiously dried in a gentle beat, so that their strength and properties maybe more completely preserved : and, if they contain any subtle or volatile matter, it will be advisable to pulverize them as speedily as possible, and to keep such powder in close glass vessels.
All fruits, however, should be allowed to become perfectly ripe, before they are removed from their stems or branches, excepting sloes, and one or two other astringents, that Jose their' virtues, if suffered to remain on the trees till they attain to maturity. Nor should seeds be collected, until they begin to grow dry, and are about to drop or shed spontaneously; when they ought to be preserved in an open situation, without being separated from their husks : as these serve to protect them from injuries of the air and weather.
Under the article Preservation, we have alluded to a new and excellent method of keeping vegetables of every description in a sapid state, for many years, excepting cucumbers and radishes: - it simply consists in drying them on a plastered floor, or an oven, moderately heated by a fire made below the structure; so as to avoid singeing or burning the leaves, stalks, etc.; the whole process being conducted in the manner about to be described, and requiring no farther care in regulating the degrees of heat, than is necessary in the baking of thin biscuits ; provided the former be exposed on their surface to the open air, for dissipating the moisture, while the latter are confined in an oven.
In order to succeed completely in this useful pursuit, the herbs and roots, as well as every species of fruit to be preserved, ought first to be cleaned, either by wiping, washing, and otherwise cleansing it in a manner similar to that practised for culinary purposes. The water should be completely drained, by placing the different articles on sieves, or frames on which canvas is expanded, or perforated boards, or similar contrivances. After repeatedly turning the leaves, stalks, or fruits, so that each side may become dry, they must be spread over a floor or oven, constructed on the principle before mentioned, till all their moisture be thoroughly evaporated; for, if the least humidity remain within the substance of such vegetables, they will become mouldy and corrupted. The best criterion for ascertaining the due degree of exsiccation, is that of the stalk breaking readily, and the leaves being easily reduced to powder between the fingers. In this shrivelled state, however, they could neither be packed nor conveniently transported : hence, it will be proper to remove them previously to a cellar, or other damp place, till they have become so pliable by the absorption of moisture, as to be compressed without crumbling to pieces. This degree of humidity is, according to experience, not detrimental to the preservation of the plants, and in its effects very different from that retained in their interior parts. Next, the Vegetables thus prepared, ought to be packed either in strong paper formed in the manner adopted with tobacco-leaves, or in wooden boxes which have been completely dried; as otherwise they will acquire the flavour peculiar to the wood. If the directions here given be strictly followed, vegetables may not only be preserved for a long time, without losing any of their essential properties, but they may also be reduced to the l6th, 20th, nay, to the 24th part of their natural bulk. Mr. Eisen, the inventor of this process, observes in his paper quoted in a former article, that half an ounce of such concentrated herbs or roots will be a sufficient allowance for each person, if eaten together with animal food ; and that a traveller may carry provisions for two months in his pockets, especially as mushrooms, lobsters, small fish, and other animal substances, may be preserved with equal success. - Lastly, when such provisions are to be dressed, it will be necessary either to infuse them for a short time in hot water, before they are exposed to the fire; or, to steep them, particularly leguminous fruits, in cold water, so that they may swell to nearly their natural size ; after which they may be treated in every respect like other culinary objects.