Acetic acid forms a considerable article of commerce. It is not only
used in medicine and the culinary art, but is extensively employed
in forming acetate of iron for dyeing and calico printing. To
prepare it, large iron cylinders, about eight feet long and three in
diameter, are embedded in brickwork in a row, and in such a manner
that furnaces placed below may heat them red-hot; these cylinders
have a tube leading from each into a main tube, where the liquid
products from each cylinder are received for condensation; the other
end of the cylinder has a plate of iron fitting closely to it. The
cylinders 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. About seven or eight
hundred weight of wood is put into each cylinder. In the morning,
the charcoal, into which the wood is now converted, is withdrawn,
and a fresh charge supplied; from this charge of wood about thirty
or forty gallons of liquid is 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
creasote; it is from this source all the creasote, so famous for the
cure of toothache, is obtained. The next process is to purify this
liquid; it is first distilled, and this separates much 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
unison with the acetic acid, is washed out from the lime and strained
quite clear; it is afterwards evaporated till it crystallizes, and
vitriol (sulphuric acid) then added; finally, the acetic acid is
distilled over, and the acid left in unison 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 colorless, transparent, and
very sour, possessing a fragrant and agreeable smell. This acid 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, to many, more agreeable as a condiment.
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 flavored 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.