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Acetic Acid

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ACID, ACETIC, 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, develop 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. t is then introduced into a large cir cular vessel a, made of iron plates rivetted together, and having at its upper part a small lateral iron cylinder ; an iron cover is closely fitted to this pot, and then it 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 4 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 diiferentin different places : in some, the condensation is effected by the air, the vapour being made to pass through • 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 ctdate 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 A 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. Wheii 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 11, under the ash-hole of the furnace. 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 ihrnished 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 carbu retted 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, arid 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 fur

nace 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.W hen 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 char coal. 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 car bonized the same year it was cut. A more simplified arrangement of apparatus than the foregoing, employed for this purpose by an eminent manufacturer in Glasgow, is described by Dr. Ure, in his Chemical Dictionary. It consists of a series of cast-iron cylinders, about 4 feet diameter, and 6 feet long, which are high horizontally in brick-work. so that the flame of one furnace may play around two cylinders. Both ends are made to project a little from the brick work. One of them is provided with a disk of cast-iron accurately fitted to it, and from the centre of this proceeds a tube about 6 inches in diameter, that enters the main tube of refrigeration. The diameter of this tube may be from 9 to 14 inches, which will vary according to the number of cylinders. The other end of the cylinder, which is called the mouth of the retort, is closed by a disk of iron, smeared round its edges with clay lute, and made fast by means of wedges. The charge of wood for a cylinder of the before-mentioned dimensions, is about 8 cwt. The hard woods, oak, ash, birch, and beech, are alone used Fir does not answer. The heat is kept up during the day-time, and the furnace is allowed to cool during the night. Next morning the door is opened, the charcoal removed, and a new charge of wood introduced. The average product of crude vinegar, called pyroligneous acid, is 35 gallons. It is much contami nated with tar, is of a deep brown colour, and has a specific gravity of 1.025. Its total weight is therefore about 300 lbs. The residuary charcoal is found to weigh no more than one-fifth of the wood employed. Hence nearly one-half of the ponderable matter of the wood is dissipated in mcondensible gases. The crude pyroligneous acid is rectified by a second distillation in a copper still, in the body of which about 20 gallons of viscid tarry matter are left from every 100. It has now become a transparent brown vinegar, having a considerable empy reumatic smell, and a specific gravity of 1.013. Its acid powers are superior to those of the best household vinegar in the proportion of 3 to 2. By re distillation, saturation with quicklime, evaporation of the liquid acetate to dryness, and gentle torrefaction, the empyreumatic matter is so completely dissipated, that on decomposing the calcareous salt by sulphuric acid, a pure, perfectly colourless, and grateful vinegar rises in distillation. Its strength will be in pro portion to the concentration of the decomposing acid. It is a common error to regard the production of vinegar from the distillation of wood as a recent dis covery. In the lifiraculuns Munch of Glauber cwho has given his name to a well-known description of salts), he says, "If this juice of wood be rectified, it may be used in the preparation of good medicines ; in mechanic arts; in the making of many fair colours from the extraction of metals, minerals, and stones; and for all things for which common vinegar is used ; yea, far more commo diously, because it much exceedeth common wine and beer vinegar in sharpness." Glauber mentions some other applications of the acid which have yet to find their way into modern practice. " If hop-poles," be says, " be dipped in the oil, it not only preserves them, but fattens the plant; and as insects abhor these hot oils, if they be applied to the bark of fruit trees, it will defend them from spiders, ants, canker-worms, and other insects ; by this means, also, rats and mice may be prevented from creeping up hovel posts and devouring the grain 1" 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 oil; 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 moisture, 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, cam phor, 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. lire's Dictionary of Chemistry.