This section is from the book "An Experimental History Of The Materia Medica", by William Lewis.
Acetum Pharm. Lond. & Edinb. Vinegar: a vegetable acid liquor, produced by fermentation; either directly from fermentable juices or infusions; or from such as have been previously fermented into a vinous state. The more spirituous the wine, the more acid is the vinegar.
Vinegar is not a pure or simple acid, like those of the mineral kingdom: in open vessels, it grows vapid, ropy, and putrid, while the mineral acids remain unchanged. Distilled by a moderate heat, not exceeding that of boiling water, it yields first a phlegmatic liquor, afterwards a slightly acid one, which is succeeded by stronger and stronger acids; till the matter in the distilling vessel becomes thick and unctuous like honey: the vinegar prepared from malt liquors contains more of this viscous sub-stance than that of wine, and hence is more disposed to become ropy and slimy in keeping. This residuum, urged with a stronger fire, gives over an empyreumatic oil, and a penetrating acid spirit tainted with the ill smell and yellow colour of the oil. There now remains a black coal, which, burnt into white ashes, yields a considerable proportion of fixt alkaline salt.
Pure fixt alkaline salt, saturated with the colourless distilled liquors, and afterwards exficated, contracts a yellowish or brown tinge; and thus betrays, that the acid still retains a portion of the oil. On gently melting the dry salt, the oily matter burns to a black coal, which separates on dissolution in water: the solution exhaled to dryness, leaves a perfectly white neutral salt, containing the pure acetous acid combined with the alkali. On adding to this compound a little oil of vitriol, the acetous acid is disengaged, and may now be collected by distillation, in a highly concentrated state, and of a very pungent volatile smell.
Acetum dis-tillatum Ph. Lond. & Ed.
The quantity of fixt alkaline salt, which vinegar is capable of saturating, is one of the surest criterions of its strength. The best of the German vinegars, according to Stahl, saturate little more than one fortieth of their own weight; the French vinegars, examined by Geoffroy, above one thirty-fifth, and some of them no less than one twelfth; the common distilled vinegar of our shops about one twentieth (a). By congelation, and by distillation from alkalies, as above-mentioned, and from some metallic bodies, particularly copper, the acid may be so far concentrated as to saturate near equal its own weight; a greater degree of strength, than even the mineral acid spirit of sea salt can easily be brought to.
(a) It cannot be affirmed that the strengths of the several vinegars examined were exactly in the proportion of the above numbers, as the alkaline salt, used by different persons or at different times, may have differed in purity or dryness, and as the common way of judging of the saturation is too vague for determining the quantity to any degree of nicety. For all trials of this kind, whether with vinegar or other acids, the alkaline salt (that of tartar is the most eligible) should be previously melted, that all remains of watery moisture may be expelled from it; and the saturation should be determined by means of coloured papers, as mentioned at the end of this article.
The London college have in their last Dis-pensatory admitted a concentrated preparation of vinegar, under the name of Acidum Acetosum, made by a simple distillation in a sand heat of verdegris coarsely powdered and first well dried in a water bath. The liquor is purified by re-distilling. Its specific weight to water is stated at 1,050 to 1,000..
The acetous acid, however purified or concentrated, differs effentially from all the others: - from the native vegetable acids, in subtility and volatility; not being obtainable in the form of a concrete salt, which most, perhaps all, of the native ones are; and rising in distillation with a moderate heat, which very few of the native ones have been found to do; most of the acid juices giving over, in the heat of boiling water, only their aqueous fluid, and having greatest part of their acidity destroyed by a stronger heat: - from the mineral acids, in its habitude to different bodies, and the nature of the compounds which it forms with them: thus, whatever alkaline, earthy, or metallic substance, the acetous acid be combined with, the addition of any mineral acid will disjoin them, the mineral taking the place of the acetous: neutral salts, composed of the acetous acid and. fixt alkalies, dissolve, totally and plentifully, in rectified spirit of wine, whilst those, composed of the same alkalies and mineral acids, are not at all soluble in that menftruum: in this property, the acetous acid differs also from most, perhaps from all, of the acids of its own kingdom: - and from all acids in general, in its peculiar odour.
Vinegar dissolves the elixated allies of vegetables, at least in great part; animal earths, purified by incineration, or when naturally blended with but little gelatinous matter, as in shells; the earth of alum; and the mineral cal-careous earths. - The solubility of calcareous earth in the acetous acid, and its precipita-bility by that of vitriol, afford a ready method of discovering the sophistication of vinegar, said to be sometimes practised, with vitriolic acid. If a saturated solution of any calcareous earth, as chalk, made in strong vinegar, be added to such as is suspected of containing vitriolic acid, no change will ensue if the vinegar was pure; but if it contained even a minute portion of that acid, the mixture will immediately become milky, and on Handing for a little time depo-fite a white sediment: if the calcareous solution be gradually dropt in, so long as it produces any milkiness or cloudiness, all the vitriolic acid will be absorbed by the chalk, and as this new compound is exceeding sparingly dissoluble, nearly the whole of it will precipitate, so as to leave the vinegar almost pure.
It dissolves, among metallic bodies, zinc, iron, copper, tin, lead, bismuth, and regulus of antimony; the two last in very small quantity, but sufficient to give a strong impregnation to the vinegar. It dissolves lead more easily when reduced into a calx, than in its metallic state: boiled even with the glass of lead, or in the common glazed earthen vessels, in the glazing of which this metal is a principal ingredient, it extracts so much as to become strongly tainted with the pernicious qualities of the lead.