This section is from the book "An Experimental History Of The Materia Medica", by William Lewis.
Vegetables should be gathered chiefly from those foils, in which they naturally delight, or in which they are found most commonly to rife fpontaneous; for, though many of them may be raised, and made to grow with vigour, in very different ones, their virtue generally suffers by the change. A variation of seasons occasions also differences considerable enough to require, oftentimes, an allowance to be made in the quantity; plants in general proving weaker, though more luxuriant, in rainy than in dry ones. - Herbs and flowers are to be gathered in a clear dry day, after the morning dew is gone off from them. Leaves, for the most part, are in their greatest perfection, when come to their full growth, just before the flowers appear: flowers, when moderately expanded: seeds, when they begin to grow dry, before they fall spontaneously; woods and barks, as is supposed, in the winter: annual roots, before the stalks begin to rife: biennial roots, in the autumn of the first year: perennial roots, in the autumn, after the leaves have fallen, or early in the spring before they begin to vegetate. To most of these rules there are some exceptions, which are spe-cified under the particular subjects.
Of the vegetables which lose their virtue in being dried, the greater number, perhaps all, may be preserved for a considerable length of time, by impeding the exhalation of their native moisture, for fo long as they retain this, they seem to retain also their medical activity. Thus roots have their virtue preserved by being buried in sand, which should be dry, that they may not vegetate: leaves and flowers, of a more corruptible nature than roots, by being beaten with about thrice their weight of fine sugar to prevent their corruption, and kept in a close vessel.
Plants which bear drying, are commonly hung in a warm airy place, defended from the fun. The colours of herbs and flowers are for the most part changed or destroyed, in drying, by the fun's beams; but that their medicinal virtue suffers a like diminution, does not appear. Thus much is certain, that a heat of culinary fire, equal to that of the fun in summer, does them no injury in either respect and that both flowers and leaves, when thus hastily dried by fire, preserve the liveliness of their colour, and their smell and taste, more perfectly than by flow exsiccacion. The leaves of moderately juicy plants are reduced, by drying, to about one fourth of their original weight.
Some roots, and some other parts of vegetables, how thoroughly foever they have been dried, are liable, in keeping, to grow mouldy and carious. This inconvenience might probably be obviated by dipping them, when dried, in boiling spirit of wine, or expofing them to its vapour in a close vessel. It is said, that some of the oriental spices are made less perishable, by being dipt in a mixture of lime and water.
The pulps of fruits are separated from the seeds and membranous parts, by forcing them through a strong hair sieve. If the fruit is unripe and hard, or if it is dry, it should be pre-viously softened by boiling in a little water; and the pulp, after passing through the sieve, is to be infpiffated over a gentle fire, with care to prevent its burning.
The concrete gummy-refinous juices brought from abroad, which have usually a considerable mixture of bits of stalks, leaves, seeds, etc. are purified by adding so much boiling water, as will so far soften or dissolve them, that they may be preffed, whilst hot, through a strainer; and then infpiffating the drained liquid, in a gentle heat, to the original consistence of the gummy-refin: if the quantity of water is considerable, the refinous part commonly separates and sub-sides, and in this case is to be kept by itself till towards the end of the infpiffation of the gummy, at which time they may be easily united again together into an uniform mass. Some of the gummy-refins, exposed to the heat of boiling water, melt thin enough, without any addition, to be pressed through a canvas strainer. In either process, the operator must be careful to prevent, as much as possible, the dissipation of the more volatile parts; an injury which cannot be wholly avoided, especially when the fub-jects are dissolved by water. The finer tears unpurified are in many cases preferable, for internal use, to those that have been strained.
Pulverable bodies of an earthy texture, or such as are brittle and not dissoluable in water, after being reduced to a powder of moderate fineness, are brought to an impalpable or very subtile state, by grinding them with a little water on some hard smooth instrument: the matter is commodiously dried on a chalk-stone, or rather on a cake of plaster-of-Paris, which equally absorbs the moisture, without adhering to the powder, like substances of the chalky kind. Powders thus levigated are still found to contain a quantity of gross parts; which may be separated by shaking the matter with water, till it is diffused through the fluid, and then suffering it to fettle: the groffer parts soon sub-side; and the turbid liquor, being now poured off, deposites more (lowly the finer powder. By this process, powders may be obtained of any degree of fineness; the tenuity being in proportion to the length of time that they remain suspended in the fluid. On the same principle, the bolar earths may be separated from the gritty matter naturally mixed with them, metallic bodies from those of the earthy kind, and the calces of metals from metallic particles uncalcined.
Salts are purified from indiffoluble admixtures, by solution in water and filtration through paper. Water dissolves, in a boiling heat, a much larger quantity of most kinds of salts than it can retain when cold: thus, of nitre, it dis-solves when boiling near three times its own weight, but in cooling, a part of the falt gradually separates, till at length, when grown thoroughly cold, in frosty weather, it does not retain one eighth its own weight, or one twenty-fourth of the quantity of falt at first dissolved. The neutral salts, or those composed of an acid and an alkali; several of those which consist of an acid and an earthy or metallic body; and many of the acid salts of vegetables; in this separation from their solutions, concrete, unless too hastily forced together by sudden cooling, or disturbed by agitation or other causes, into transparent masses, of regular figures peculiar to each particular kind of salt, and thence called crystals. - There are two general methods of recovering salts from their solutions in a crystal-line form; one adapted to some salts, and the other to others. The one is, by keeping the solution in a gentle and equable warmth, that the water may gradually exhale, and leave the falt crystallized. The other is, by boiling down the solution, till, on dropping a little of it on a cold glass plate, crystalline filaments appear; then covering the vessel, and suffering it to cool very slowly. Some of the difficultly crystallizable salts are made to shoot more freely, by adding, after sufficient evaporation, a small proportion of rectified spirit of wine, which weakens the dissolving power of water on most kinds of saline bodies. - As different salts require different quantities of water to keep them suspended; when two or more are dissolved together, they begin to concrete at different periods of the evaporation, that, which requires most water for its dissolution, fhooting first, and leaving the more soluble dissolved: on this foundation, salts are purified, by crystallization, from admixtures of one another.