When the immediate products of the vinous fermenta tion are left exposed to the air, even at the ordinary tem peratures of the atmosphere, they undergo another change, and arc converted, by degrees, into an acid liquor. This forms the second stage of the general process of fermen tation ; and is termed acetous, because it produces acetous acid or vinegar. This species of fermentation commences immediately after the vinous stage is completed; and more especially if the fermentative principle has been used in excess. Hence, after the termination of the latter, the fer mentation is usually stopped by decanting the clear wine from the lecs, clarifying it with isinglass, corking it up carefully, and placing it in a cool situation, where it may remain undisturbed. By these processes, the yeast, which may still exist in excess in tile vinous product, is almost entirely removed, and thus the fermented liquor is not ex posed to any decomposition from the reaction of its own elements.
The acetous fermentation is not confined to the products of the vinous stage; for it appears, that some substances not susceptible of vinous fermentation, as fecula and mu cilage, have an ascescent tendency ; but the bodies which have passed through that process, as wines, cider, beer, &c. arc most susceptible of spontaneous acetification, and the richest and most generous wines furnish the best and strongest vinegar.
The vinous liquors do not readily undergo the acetous fermentation without the assistance of some fermentative principle; and hence, on converting wines into vinegar, a quantity of that substance, in some form or other, is usu ally added. It is in this way that the lees of vinegar, and casks impregnated with it, decide and promote acetifica tion.
The presence of air is no less necessary to the acetous fermentation. Wines well corked in bottles, and grapes properly closed up in casks, may be preserved a very long time without suffering much change; but if the air he imperfectly excluded, they are gradually rendered sour ish, and that, in a greater degree, the more freely the air is admitted. Saussure states, that in this case the oxygen of the air enters into combination with the carbon of the vinous liquor, and abstracts that element from it in the form of carbonic acid. It is probable, however, that a portion of it also combines with the same substance, and contri butes to the production of the acetic acid which is formed.
The acetous fermentation goes on when the tempera ture is below 60°, but it is then slow ; and experience has proved, that the process advances best when the tempera ture is between 70° and of Fahrenheit's scale. In ma nufactories for making vinegar, the heat is kept up at this pitch by artificial means, when the temperature of the air is too low.
A slight agitation, repeated at intervals, is extremely fa vourable to acetification. For this reason, cellars which are exposed to the continual shaking produced by any powerful mechanical instrument, or to the tremulous mo tion excited by the frequent rolling of heavy carriages, are very unfit for preserving vinous liquors. By the agitation to which wine is in such cases exposed, the tartar, the lees, the extractive principle, and all the other substances which it deposits in a state of rest, being kept suspended, operate incessantly as so many ferments, and prevent the depura tion which might otherwise take place. Hence the ace tous fermentation is greatly promoted by frequently stir ring the liquor exposed to its action.
The general appearances which present themselves in the acetous fermentation, differ but little from those in the vinous. A tremulous movement pervades the whole mass; but this is attended with a less copious disengagement of carbonic acid than in the vinous stage of the process. The temperature rises, and if the quantity of liquor be considerable, sometimes reaches 90° of Fahrenheit's scale. In the mean time a kind of filaments or streaks are moving continually in the heart of the fermenting mass ; these di vide, reunite, and at last deposit themselves on the sides and bottom of the vessel. When all these phenomena
have ceased, the liquor gradually recovers its transparency, and is found to be converted into vinegar.
We shalt now make a few observations on the theory of the process, though on this head we can offer nothing very precise or satisfactory. According to Lavoisier, the spi rituous part of the wine, which consists of carbon and hy drogen, is oxygenated and converted into vinegar. This operation, he adds, can only take place with free access of air, and is always attended with a diminution of the air em ployed, in consequence of the absorption of oxygen." This explanation is too general to he satisfactory; and it does not at all bring into view the action of the ferment. Chap tal has given a theory or the process, which is less excep tionable. ec The hydrogen and the carbon, says he, exist in alcohol, and in the extractive principle of vegetables ; but hydrogen predominates in the former, and carbon in the latter ; so that if we oxygenate them separately, alco hol would furnish plenty of water, and very little acetic acid. The extractive prinCiple would furnish plenty of carbonic acid, and a little acetic acid. But when the two principles are united, and they are oxygenated by any pro cess whatever, water and carbonic acid are then produced, which bring the two principles into the proportions proper for forming the acetic acid." The vinous and acetous fermentation are confined to a very few substances, chiefly of a saccharine nature : the putrefactive stage embraces a wider field, and takes place in almost every body of a vegetable or animal nature. The vegetable matters which undergo putrefaction most rea dily, are soluble in water; though those which are but im perfectly soluble, if kept in a moist state, are not exempt ed from this species of decomposition. This process is promoted by the same circumstances which arc favourable to the others, namely, moisture, and elevation of tempera ture. The presence of air, also, has no less influence on the putrefactive, than on the acetous stage.
The elastic fluids which are evolved from vegetables during the putrefactive fermentation, are combinations of the elements of the vegetable substance, and have for their bases hydrogen and carbon. When the decomposition takes place under water, the hydrogen, by its greater ten dency to elasticity, makes its escape, and the residual mat ter consists almost entirely of carbon. hence wood, which has been long buried in the beds of rivers, is reduced near ly to the state of charcoal. If the carbonaceous part, how ever, be exposed to the air, it undergoes a gradual change, and is at last entirely decomposed, by being converted into carbonic acid.
When animal matters suffer putrefaction, they evolve, besides the usual elements of vegetables, a quantity of ammonia. They yield also certain other products which are more peculiar to them, particularly combinations of sulphur and phosphorus ; and to these substances must be ascribed, the fetid odour and noxious properties of the gases, which are extricated from them during putrefac tion.
Animal bodies scarcely suffer any change when they are well dried, and completely excluded from the air. Even in the warmer climates, beef, which has been effectually freed from its juices, may be preserved a long time with out salt; and meat, which has been sufficiently roasted, and afterwards covered with melted suet, may be preserv ed in that state perfectly untainted for several months. Animals enveloped in ice, have been preserved for ages without suffering any change. It appears, also, that ani mal bodies powerfully resist putrefaction, which have been buried in morasses of peat; probably because, in such places, the carbonaceous part of the woody matter being converted into a substance resembling tan, produces upon the animal matter the usual effects of that vegetable pro duct. See Chaptal's Chemistry, vol. iv. p. 510 ; Murray's Chemistry, vol. iv. p. 387 ; Fourcroy's Chemistry, vol. ii. p. 302. (A)