Acetic acid, or “wood vinegar,” is prepared as follows:—There are some large iron cylinders set in brickwork over furnaces, and these cylinders have each a tube leading to a main pipe in which the liquid is received for condensation. The cylinders, which contain about seven or eight hundredweight, are filled with logs of wood, either oak, beech, birch, or ash, the door is closely fastened, and the joints smeared with clay; the fires are now lighted and kept up all day, till the cylinders are red-hot; at night they are allowed to cool. In the morning, the charcoal, into which the wood is now converted, is withdrawn, and a fresh charge supplied; it is then found that about thirty or forty gallons of liquid has condensed in the main tube from each cylinder, the remainder being charcoal and gases which pass off; the liquid is acid, brown, and very offensive, and contains acetic acid, tar, and several other ingredients, among which may be named creosote; it is from this source all the creosote, for the cure of toothache, is obtained. To purify this liquid it is first distilled, and this separates much of the tar; it is then mixed with lime, evaporated to dryness, and heated to expel the remaining tar and other impurities; it is next mixed with sulphate of soda and water, and the whole stirred together; the soda, now in union with the acetic acid, is washed out from the lime and strained quite clear; it is afterwards evaporated till it crystallises, and vitriol (sulphuric acid) then added; finally the acetic acid is distilled over, and the sulphuric acid left in union with the soda, forming sulphate of soda, to be used in a similar process for the next batch of acid. The acetic acid is now quite colourless, transparent, and very sour, possessing a fragrant smell. This is not pure acetic acid, but contains a considerable quantity of water. The acetic acid of commerce, mixed with seven times its bulk of water, forms an acid of about the strength of malt vinegar, perfectly wholesome, and agreeable as a condiment.
Fig. 422.—Vinegar-cooling process.
Pure acetic acid may be made by mixing dry acetate of potash with oil of vitriol in a retort, and distilling the acetic acid into a very cold receiver; this, when flavoured with various volatile oils, forms the aromatic vinegar sold by druggists. It is a very strong acid, and if applied to the skin will quickly blister it.
Fig. 423.—Tan-yard and pits.
Acetate of lead, or sugar of lead, is obtained by dissolving oxide of lead in vinegar. A solution of this salt makes the goulard water so familiar to all. Acetate of lead is highly poisonous.
Acetate of copper is verdigris, and poisonous. Other acetates are used in medicine.
We may pass quickly over some other acids. They are as follows:—
Tartaric Acid (C4H6O6) is contained in grape juice, and crystallizes in tabular form. The purified powdered salt is cream of Tartar.