The constituents of vegetable matter, when separated from the living plant, and placed under certain circumstances, act upon one another, and a spontaneous decomposition takes place, even at the ordinary temperature of the atmosphere. This process has been denominated Fermentation by chymists, on account of the intestine motion with which it is accompanied; and, as its phenomena and results vary according to the nature of the vegetable matter subjected to it, and the circumstances under which it occurs, the general process is divided into three species easily distinguished from one another. The 1st is named the vinous fermentation-of which the products are wine, beer, and other vinous fluids : the 2d, the acetous fermentation, which produces acetic acid of vinegar; and the 3d, the putrefactive fermentation, in which gases, chiefly foetid, are produced, and ammonia.

Each of these is occasionally artificially produced for pharmaceutical purposes, and therefore requires to be described.

Vinous fermentation. All vegetable substances containing saccharine matter, and a peculiar glutinous principle analogous to the gluten of wheat, are susceptible of this fermentation. For its commencement, however, the presence of water, sugar, extract, and a small proportion of vegetable acid, with a certain increased temperature, is requisite. In juices in which these are present, the fermentation is spontaneous; but, as yeast contains the peculiar gluten, and the other principles necessary for exciting the vinous fermentation in any sweet vegetable juice or decoction, it is frequently used for this purpose in the formation of beer and wines. Soon after yeast is added to these substances, or to wort1, or to must2, an intestine motion commences in the liquor, its temperature rises, it becomes turbid, and carbonic acid gas is .extricated, and a scum, or hat, as it is termed, is formed on the surface of the liquid : but, after some time, the fermentation again gradually subsides, the scum which was formed during its continuance sinks to the bottom; the liquor becomes lighter, and instead of its sweet taste has acquired that peculiar taste and flavour which is denominated vinous.

This process of fermentation is never employed in the laboratory for the preparation of vinous liquors, although these are articles of the materia medica: but the cataplasms, which are prepared from carrots and similar vegetables mixed with yeast, derive their virtues from the vinous fermentation into which they enter, extricating a large quantity of carbonic acid gas, which operates as a powerful sedative antiseptic.

Acetous fermentation. All liquors prepared by the vinous fermentation are susceptible of the acetous when kept exposed to the air in a temperature between 70° and 90°. Under these circumstances the liquor gradually becomes thick, its temperature increases, and filaments are seen moving through it in every direction, an intestine motion being excited, accompanied with a hissing noise: but as this motion subsides, these filaments fall to the bottom, or attach themselves to the sides of the vessel: the liquor becomes clear and transparent, and has acquired a very sharp acid taste. In this state it is denominated vinegar, and contains, besides the acetic acid and water, which are its principal components, mucus, malic acid, supertartrate of potassa, and some other vegetable constituents.

1 Wort is an aqueous infusion of malt. It consists of saccharine matter, starch, gluten, tannin, and mucilage.

2 Must is the expressed juice of the grape. It contains water, sugar, a peculiar matter which changes into gluten by contact with the air, mucilage, supertartrate of potassa, tartrate of lime, muriate of soda, and sulphate of potassa.

Pure alcohol, even when diluted with water, is not susceptible of this fermentation, but it enters into it when united with other fermentable bodies : thence wine and vinous liquors, which contain, besides alcohol, sugar and some mucilaginous and extractive matter, are employed for making vinegar. In this process the alcohol is supposed to be decomposed, and oxygen absorbed: carbonic acid is formed, but is retained in the liquor; and it is probable, as Dr. Murray suggested, that the ferment affords the nitrogen, which enters into the composition of the acetic acid.1

Many vegetable infusions and decoctions undergo this fermentation in warm weather; and thence the necessity of preparing these every day during summer, as by the decomposition which takes place, their medicinal virtues are completely destroyed, when they are kept.

Putrefactive Fermentation

Putrefactive Fermentation. Almost every vegetable product kept in a moist place, and in a temperature not under 45°, nor above 70°, undergoes spontaneous decomposition. The solid structure of the body is completely destroyed, and its ultimate principles, entering into new combinations, escape in the gaseous or aeriform state, leaving behind a small quantity of earthy and metallic matter only, which it contained.

This process, which is denominated the putrefactive fermentation, does not absolutely require the contact of air; but water, in every case, appears to be essential. Vegetable bodies, which are very soluble in water, most readily undergo it; the surface of the liquor becomes covered with a mould; various elastic fluids, in which ammonia and phosphureted hydrogen gas are often perceptible, with other matters which produce a foetid odour, are extricated, and the vegetable matter is ultimately completely decomposed.

1 System of Chymistry, iv. 465.

The knowledge of the circumstances which promote this species of spontaneous decomposition points out the necessity of preserving vegetable substances in perfectly dry places and, when they have a tendency to attract moisture, of exposing them in a free current of air to dissipate the humidity which they would otherwise absorb.

c. Of the operations in which oxidizement and deoxidizement are effected by means of a high temperature.

The degree of temperature at which these operations are conducted cannot generally be obtained from a common fire; and therefore, before describing the operations themselves, it is necessary to notice the nature of furnaces, which are instruments of the most universal use in pharmaceutical chymistry.