Acetic Acid, is the acid contained in common vinegar, but in a very dilute state, and in combination with other vegetable principles. It is found united with potash in a great variety of plants, also in several animal secretions. It is likewise the result of a spontaneous fermentation, to which liquid, vegetable, and animal matters, are liable. Strong acid, as the sulphuric and nitric, develope the acetic by their action on vegetables. Dry vegetable substances generally, when subjected to a red heat in close vessels, yield it copiously. The proportion of the products varies not only from employing different substances, but they are different when only one substance is employed, according as the heat is greater or less, or the operation is differently managed. When a vegetable substance is distilled in close vessels, at first the water comes over which existed ready formed, and then water formed by union of the oxygen and hydrogen of the substance. Afterwards, a quantity of carbon is separated; and by the continued application of heat, this unites with the oxygen and hydrogen, and forms an acid, formerly supposed to be a particular acid, and then called pyroligneous acid, but it is now known to be the acetic acid, united with empyreumatic oil, which rises somewhat brown, and grows thicker and darker, augmenting in density as the quantity of carbon increases.
At the same time, a small quantity of carbonic acid gas, much carburetted hydrogen, and, towards the close, a great quantity of gaseous oxide of carbon, are disengaged. All the carbon not carried off in these various forms remains in the still, and generally preserves the form of the vegetable substance employed. Since we have learned the nature of all these products, the process has been much improved, and particularly by charring the wood, and by turning the other products to advantage. In the forests, the wood is first charred, so as to dissipate all the water of vegetation. It is then introduced into a large circular vessel, a, made of iron plates riveted together, and having at its upper part a small lateral iron cylinder; an iron cover is closely fitted to this pot, and then is lifted by means of a crane, or other mechanical power, and placed in a cast-iron retort, c, set over a furnace of the same shape. The furnace is then covered with a lid, e, constructed in masonry. A moderate heat being applied to the furnace, at first the vapour of the wood is dissipated, but this vapour soon ceases to be transparent, and becomes sooty At this time a tube or cylinder, enclosed in another of brick-work or tiles, is affixed to the lateral cylinder, and forms the condensing apparatus.
This is different in different places: in some, the condensation is effected by the air, the vapour being made to pass through a long extent of cylinders, and sometimes of casks, adapted to each other; but most generally the condensation or cooling is effected by water, when it can be procured in sufficient quantities. The most simple apparatus for this purpose consists of two cylinders f f, enclosed one within the other, and having between them a space sufficient to allow a large quantity of water to flow backwards and forwards, and thus cool the vapour. These cylinders are adapted to the distilling apparatus, and placed inclined to the horizon. To this first apparatus a second, and sometimes a third is adapted, and placed in a zig-zag form, in order to occupy as little space as possible. The water is made to cir culate in the following manner.
At the lower extremity g of the condensing apparatus, there is a tube which ought to be somewhat higher than the upper part of the whole of this apparatus, where at h there is another short tube curved towards the ground. Water from a reservoir is made to run through the perpendicular tube to the lower part of the condensing apparatus, and fills all the space between the cylinders. When the operation is going on, as the vapours are condensed, they raise the temperature of the water, which, becoming lighter in consequence, flows out of the curved tube h. The condensing apparatus terminates in a covered brick canal i, underground, at the end of which is a bent tube k, that conducts the liquid products into the first cistern; when this is full, it discharges itself by means of a syphon into a large reservoir; the tube which terminates the canal plunges into the liquid, and thus cuts off the communication with the interior of the apparatus. The gas hereby disengaged is conveyed by means of the tube l l under the ash-hole of the furnace.
This tube is furnished with a cock, to regulate the flow of the gas, and cut off the communication at pleasure.
That end of the tube which terminates in the ash-hole, rises a few inches perpendicularly, and is furnished at its extremity with a perforated rose, for distributing the gas uniformly under the vessel, without being itself liable to become choked with the ashes, or to obstruct the feeding of the fire. The degree of heat necessary to effect carbonization is not very great, yet, towards the end of the process it must be raised sufficiently to make the vessel red-hot, and the length of the operation is necessarily regulated by the quantity of wood to be carbonized at the time. By the colour of the gas flame it is ascertained when the carbonization is complete; at first it is of a reddish yellow colour, but afterwards it becomes blue, as it throws off more oxide of carbon than carburetted hydrogen; at last it becomes entirely white, probably caused by the vessel being hottest at this period, and the combustion, therefore, may then be considered as quite finished. There is also another method of ascertaining the completion of the operation, which is more frequently had recourse to; that is, the cooling of the upper part of the tubes,.which is not surrounded with water; some drops of water are then thrown upon it, and if these evaporate without noise, the operation is considered to be finished.
The adapting short tube is next removed from the vessel, and the opening into it immediately closed by an iron-plate cover, which is then luted with loam. The lid which covers the furnace is next removed, and then the vessel itself is lifted out of the furnace by means of the crane, which should be immediately replaced by another similar retort got ready for the purpose. When the retort which has been taken out has become cold, it is uncovered, and the charcoal taken out. Whatever may be the kinds of wood employed in this operation, nearly the same results are obtained, as far as respects the acid; not so, however, with regard to the charcoal. The denser the wood, the better the charcoal; and it has been remarked, that when the wood has been long left in contact with the open air, the charcoal produced from it is of a much worse quality than from that wood which is carbonized the same year it was cut. An acetic acid of very considerable strength may also be prepared by saturating perfectly dry charcoal with common vinegar, and then distilling.
The water easily comes off, and is separated at first, but a stronger heat is required to expel the acid.
If vinegar be exposed to very cold air, or to a freezing mixture, its water will be separated in the form of ice, and the interstices be filled with a strong acetic acid, which may be collected by draining. The radical vinegar of the apothecaries, made by dissolving in it a little camphor, or fragrant essential oil, has a specific gravity of about 1.070, and consists of one part of water to two of the crystallized acid. The pungent smelling salt is made by moistening the sulphate of potash with a little concentrated acetic acid. Acetic acid acts upon iron, zinc, copper, and nickel, in the metallic state, and upon the oxides of various other metals; its combination with the latter being usually effected by mixing a solution of their sulphates with that of an acetate of lead. It has a very slight action upon metallic tin, when highly concentrated. The strongest acetic acid will, we are informed, dissolve metallic lead, which is contrary to the statements of chemical authors. Acetic acid dissolves resins, gum-resins, camphor, and essential oils. Its odour is employed in medicine to relieve nervous headache, fainting, and sickness from crowded rooms. Its anti-contagious powers are not now so confidently relied upon as formerly. It is extensively used in calico-printing.
It unites with all the alkalies, and most of the earths, and with these bases it forms compounds, some of which are crystallizable, and others have not yet been reduced to a regularity of figure. For the properties and uses of these combinations we refer the reader to Dr. Ure's Dictionary of Chemistry.