Or the following process may be adopted for the separation of the lead :-Mix the concentrated nitric acid solution with hydrochloric acid in excess ; add a large quantity of absolute alcohol mixed with some ether; let the precipitate subside, filter the fluid off, wash the precipitate with alcohol and ether, dry it at a gentle heat, and weigh.

The following process for detecting and estimating minute quantities of oxide of copper is by Mr. Warington :

"This operation depends upon the solubility of the ferrocyanide of copper in an excess of a solution of ammonia, and its deposition with

its well-marked characteristic appearances as the ammonia evaporates. Thus, supposing a frequently occurring case, where the oxide of copper, in very small quantity, is in solution with oxide of iron, and that these metals have been brought to their highest state of oxidation; ammonia is next added in excess, and then a few drops of a solution of the ferrocyanide of potassium, and the whole thrown upon a filter. As the ammonia escapes from the filtrate by standing and free exposure to the air, the red ferrocyanide of copper will be deposited, and if the experiment be made in a shallow white porcelain dish, the

result will be very distinct and characteristic, and on carefully decanting the fluid, the precipitate will be found on the white surface. In many cases, the process of filtration may be dispensed with altogether, as the suspended peroxide of iron does not in the least interfere with the deposition of the ferrocyanide of copper from the solution. I have found this test gives unerring indications in cases where no trace of blue colour could be distinguished in the ammoniacal solution, and where no precipitation could be procured by hydrosulphuric acid gas or the action of a voltaic circuit.

* When organic colouring matter is present, this form of test is also very useful, as in vinegars, &c.'

CHAPTER XXXVIII.

VINEGAR AND ITS ADULTERATIONS.

DEFINITION OF ADULTERATION.

Free sulphuric acid beyond the quantity allowed by law, or any other mineral acid, or vegetable acid, excepting acetic acid, derived from the manufacture of the vinegar. Water in such proportion as to reduce the acetic acid to below 3.5 per cent. It is questionable whether it would not be well that the law should be abolished which allows of the addition of 1 part of sulphuric acid in 1,000 parts of vinegar.

ACETIC acid is the volatile principle, to the presence of which, diluted with variable proportions of water, vinegar mainly owes its aroma and pungency.

This acid exists, ready formed, in notable quantity in certain plants, as Sambucus niger or black elder, Phoenix dactylifera or Date tree, and Rhus typhenus.

It may be readily generated by the fermentation of various vegetable and animal substances, especially the former.

For commercial purposes it is made from certain vegetable and spirituous infusions, as those of the grape, malt, and the sugar cane; but any vegetable infusion capable of yielding alcohol will also, when exposed to the necessary conditions, furnish vinegar. In most cases, when vinegar is manufactured on a large scale, the vinous or alcoholic fermentation precedes the acetous, and the vinegar is formed entirely at the expense of the alcohol.

Acetic acid may be formed directly from the vapour of crude alcohol or spirits of wine in contact with the atmosphere, either by means of an ignited platinum wire, or of the black powder obtained by boiling proto-chloride of platinum and potash with alcohol. In Germany, where the price of alcohol is very low, vinegar has been manufactured on a large scale on this principle. The process will be found described at page 368 of 'Food and its Adulterations.'

Certain conditions are either essential to acetification, or else promote greatly the rapidity of the process; thus the presence of atmospheric air or oxygen is one of the conditions indispensable to the change, the reason of which will appear from what follows.

Common or ethylic alcohol, C,HO, by the absorption of 2 atoms

of oxygen from the air, yields acetic acid, C,H,O,, and one molecule of water. This process takes place in two stages. First one atom of oxygen deprives the alcohol of 2 atoms of hydrogen, aldehyde, C,H,O, being formed. This in its turn absorbs 1 atom of oxygen and yields directly acetic acid; usually, however, especially when the access of air is limited, a portion of the aldehyde escapes oxidation into acetic acid. But in most cases small quantities of acetic ether and other volatile substances are produced, giving to the vinegar much of its flavour and aroma.

Such are the changes of alcohol into acetic acid.

It is therefore evident that much of the success of any process adopted for the manufacture of vinegar will depend upon the manner in which the mother liquor is exposed to the atmosphere, that is, upon the constant renewal of the air, and the extent of surface exposed to its action, the conversion of alcohol into acetic acid taking place only on tbe surface of the liquid.

A second condition which greatly facilitates acetification is the presence of a ferment.

In vegetable infusions, as those of the grape and malt, the nitrogenised principles contained in them act as the ferment. Vinegar itself, vinegar yeast, lees, beer yeast, leavened bread, and many other similar matters, are capable of exciting fermentation, and so promoting the generation of acetic acid. Experience has shown that the best ferment for inducing the transformation of alcohol into acetic acid is a portion of ready-made vinegar itself.

A third condition, which, though not, like the former, essential, yet greatly hastens the conversion, is an increased temperature, varying considerably in different cases, but being often about 38° C.

When any spirituous liquor is exposed under the conditions requisite for acetitication, the following phenomena are observed to occur in succession.

However clear the liquid may have been at first, it quickly becomes turbid ; currents or movements are soon visible in it; it is said, in common language, to be on the work.' Slimy particles collect on the surface, gradually forming a scum, which, after a time, falls as a sediment to the bottom. The Germans call this scum vinegar mother, since it is capable of exciting acetification in fresh portions of liquid.

During the process the temperature of the liquid rises, and the peculiar aroma of vinegar becomes diffused in the surrounding air ; as soon as all the alcohol has become converted into acetic acid, the temperature falls to that of the atmosphere; the motion ceases, the liquid becomes clear and bright, and its conversion into vinegar is complete.

The different kinds of vinegar may, according to their source, be divided and classified as malt, wine, cider, beet, sugar, and wood vinegars, but occasionally other fruits, as gooseberries and currants, are employed for the manufacture of vinegar.

Malt vinegar.-An infusion of malt is prepared in the same way as the wort for the brewing of beer-namely, by exhausting the crushed malt with successive quantities of water, which has previously been heated to about 70°C. The solution is allowed to cool to about 25°C., and to 100 gallons from 3 to 4 gallons of yeast are added.

There are two essentially different methods of the further treatment of this liquid. Formerly it was filled into casks which were placed in rows in an open field, which were allowed to remain there for many weeks, until the acetification was complete. This process, called fielding, was begun in the early months of the year, and was not completed till the autumn. The casks are filled from the top of the brewhouse by means of a flexible pipe. After the completion of the acetification, the casks are emptied by means of a syphon into a trough placed beneath, and the liquid is pumped by means of steam up to the brewhouse and into the refining casks. These casks are filled with stalks and skins of grapes, or with wood shavings, straw, or spent tan, and the vinegar is allowed to filter several times through these substances. By this process the vinegar is freed from nitrogenous miatters, and rendered clear and bright. It is then pumped into vats, where it remains until it is filled into casks for sale.

The other method of acetification of the wort, which has now superseded the fielding process, is as follows:-Larye casks placed in three or four ranges over each other in a room or cellar warmed from about 28 to 30° C. are filled to about one-third of their capacity with ready-made vinegar, to which about 2 gallons of wort are added. Every week an addition of two gallons more is made. After four weeks 8 gallons of vinegar are drawn off, and, as before, wort is added two gallons at a time to the liquid in the casks. These casks are pierced with two holes, one at the top to allow of the insertion of a funnel to fill them, the other is at the side, a little above the surface of the liquid, to permit the escape of the carbonic acid formed during the fermentation of the sugar and the entrance of air.

More than two-thirds of the capacity of the cask should not be filled with the liquid, so as to expose as large a surface as possible to the action of the air. These casks are used for many years successively.

Malt vinegar prepared by either of the above described processes possesses a yellowish-red colour, an agreeable aromatic odour, and a strongly acid taste. We believe, however, that at the present day vinegar is rarely made from malt only, but from a mixture of malt with other grains, and even with sugar and treacle.

Wine vinegar.-From both red and white wines of inferior quality vinegar is prepared in wine growing countries, especially in France. The method employed for its manufacture is in its details nearly the same as that described under malt vinegar-namely, the acetification is carried on in heated rooms, a portion of the liquid being from time to time drawn off from the casks, whilst an equal addition of wine is

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