ACETUM, an agreeable acid and penetrating liquor, prepared from wine, cider, beer, and other liquors; of considerable use, both as a medicine and a sauce. The word is French, vinaigre; formed from vin, "wine;" and aigre, " sour." See ACETUM, and CHEMISTRY-Index.

Wine and other vinous liquors are changed into vinegar by the acetous fermentation. The acetous fermentation is nothing more than the acidification or oxygenation of wine, produced in the open air by means of the absorption of oxygen. Vinegar is composed of hydrogen and carbon, united together in proportions not yet ascertained, and changed into the acid state by oxygen. As vinegar is an acid, we might conclude from analogy, that it contains oxygen; but this is put beyond doubt by direct experiments. In the first place, we cannot change wine into vinegar without the contact of air containing oxygen; secondly, this process is accompanied by a diminution of the air in which it is carried on from the absorption of its oxygen; and, thirdly, wine may be changed into vinegar by any other means of oxidation. Independent of the proofs which these facts furnish of the acetous acid being produced by the oxygenation of wine, an experiment made by Mr Chaptal, professor of chemistry at Montpelier, gives a distinct view of what takes place in this process. He impregnated some water with about its own bulk of carbonic acid gas, procured from beer vats in fermentation; and placed this water in a cellar, in vessels communicating with the air, and in a short time the whole was converted into acetous acid. This carbonic acid gas, procured from beer vats in fermentation, is not perfectly pure, but contains a great quantity of alcohol in solution; wherefore water impregnated with it contains all the materials necessary for forming the acetous acid. The alcohol furnishes hydrogen and one portion of carbon; the carbonic acid furnishes oxygen and the rest of the carbon; and the air of the atmosphere furnishes the rest of the oxygen necessary for changing the mixture into acetous acid. From this observation it follows, that nothing but hydrogen is wanting to convert carbonic acid into acetous acid; or, more generally, that by means of hydrogen; and according to the degree of oxidation, carbonic acid may be changed into all the vegetable acids; and, on the contrary, that, by depriving any of the vegetable acids of their hydrogen, they may be converted into carbonic acid.

The process indicated by Boerhaave for making vinegar is still the most frequently used. It consists in fixing two casks in a warm room or place. Two false bottoms of basket-work are fixed at a certain distance from the bottom, upon which the refuse of grapes and vine twigs are placed. One of these tuns is filled with wine, and the other only half filled. The fermentation begins in this last; and, when it is in full action, it is checked by filling the cask up with wine out of the other. The fermentation then takes place in the last-mentioned cask, that remained half filled; and this is checked in the same manner by pouring back the same quantity of liquid out of the other; and in this way the process is continued till the vinegar is made, which is usually in about 15 days. When the fermentation develops itself, the liquid becomes heated and turbid; a great number of filaments are seen in it; it emits a lively smell; and much air is absorbed, according to the observation of the Abbé Rozier. A large quantity of lees is formed, which subsides when the vinegar becomes clear. This lees is very analogous to the fibrous matter.

Vinegar is purified by distillation. The first portions which pass over are weak; but soon afterwards the acetous acid rises, and is stronger the later it comes over in the distillation. This fluid is called distilled vinegar; and is thus cleared of its colouring principle, and the lees, which are always more or less abundant. Vinegar may likewise be concentrated by exposing it to the frost. The superabundant water freezes, and leaves the acid more condensed.

Method of making Cyder Vinegar.—The cyder (the meanest of which will serve the purpose) is first to be drawn off fine into another vessel, and a quantity of the must of apples to be added: the whole is set in the sun, if there be conveniency for it; and at a week or nine days end it may be drawn off.

Method of making Beer Vinegar.—Take a middling sort of beer, indifferently well hopped; into which, when it has worked well and grown fine, put some rape, or hulls of grapes, usually brought home for that purpose: mash them together in a tub; then letting the rape kettle, draw off the liquid part, put it into a cask, and let it in the sun as hot as may be; the bung being only covered with a tile, or slate-flone: and in about 30 or 40 days it will become a good vinegar, and may pass in use as well as that made of wine, if it be refined, and kept from turning muddy.

Or thus.—To every gallon of spring water add three pounds of Malaga raisins; which put into an earthen jar, and place them where they may have the hottest sun from May till Michaelmas; then pressing all well, turn the liquor up in a very strong iron hooped vessel, to prevent its bursting: it will appear very thick and muddy when newly pressed; but will refine in the vessel, and be as clear as wine. Thus let it remain untouched for three months before it is drawn off; and it will prove excellent vinegar.

To make Wine Vinegar.—Any sort of vinous liquor being mixed with its own faces, flowers, or ferment, and its tartar first reduced to powder, or else with the acid and astringent stalks of the vegetable from whence the wine was obtained, which hold a large proportion of tartar; and the whole being kept frequently stirring in a vessel which has formerly held vinegar, or set in a warm place full of the fleams of the same, will begin to ferment anew, conceive heat, grow four by degrees, and soon after turn into vinegar.

The remote subjects of acetous fermentation are the same with those of vinous; but the immediate subjects of it, are all kinds of vegetable juices, after they have once undergone that fermentation which reduces them to wine: for it is absolutely impossible to make vinegar of must, the crude juice of grapes, and other ripe fruits, without the previous assistance of vinous fermentation.

The proper ferments for this operation, whereby vinegar is prepared, are, 1. The faces of all acid wines. 2. The lees of vinegar. 3. Pulverized tartar, especially that of Rhenish wine, or the cream or crystals thereof. 4. Vinegar itself. 5. A wooden vessel well drenched with vinegar, or one that has long been employed to contain it. 6. Wine that has often been mixed with its own faces. 7. The twigs of vines, and the stalks of grapes, currants, cherries, or other vegetables of an acid astringent taste. 8. Bakers leven, after it is turned acid. 9. All manner of ferments, compounded of those already mentioned.

Vinegar Concentrated. See Chemistry, no 881.

Vinegar (Salt of). See Chemistry, no 882.