By far the most dangerous impurities in vinegar are copper and lead. The former may be detected by a brownish precipitate on the addition of ferre-cyanuret of potassium to the concentrated vinegar. The latter by a blackish precipitate with sulphuret-ed barium, and a yellow one with iodide of potas-siurn.

Pure vinegar is not discolored by sulphureted hydrogen.

The essential ingredients of pure vinegar are acetic acid and water; but, besides these, it contains various other substances derived from the particular vinous liquor from which it may have been prepared. Among these may be mentioned coloring matter, gum, starch, gluten, sugar, a small portion of alcohol, and frequently malic and tartaric acids, with a minute proportion of alkaline and earthy salts.

The method pursued in making Wine Vinegar in ounce, where it is manufactured in the greatest perfection, is as follows: Casks are employed of about the capacity of eighty - eight wine gallons; those being preferred which have been used for a similar purpose. They are placed upright in three rows, one above the other; each cask having an opening at the top of about two inches in diameter. In summer, no artificial heat is required; but the wine intended to be converted into vinegar is kept in separate casks containing beech shavings, on which the lees are deposited. Twenty-two gallons of good vinegar, boiling hot, are first introduced into each vinegar cask, and at the end of eight days about two gallons of the wine, drawn off clear, are added; and the same quantity is added every eight days until the casks are full. After this the vinegar takes about fifteen days to form. At the end of that time only half the contents of each cask is drawn off; and it is filled up again by the addition of two gallons of wine every eight days as at first. In some cases, however, the quantity of wine added, and the intervals between the successive additions, are greater or less than those here indicated. The variations in this respect depending upon the progress of the fermentation to determine this point, the operator plunges a stave into the cask, and upon withdrawing if they find it covered with froth, they judge that the fermentation is going on properly, and accordingly add more wine.

When the infusion of malt is employed in the manufacture of vinegar, the process is as follows: The infusion of malt, when properly cooled, is put into large fermenting tuns, and by the addition of yeast the liquid is fermented for four or five clays. It is then distributed into smaller vessels, and placed in a room heated by means of a stove, and kept there for about forty days, or until the mass has soured. It is then transferred to common barrels, which are placed in the open air, the bung-holes being covered with a tile to keep out the rain. In this situation they are allowed to remain for several months, or until vinegar is formed.

The process is then completed in the following manner: Large tuns are prepared with false bottoms, on which is put a quantity of the refuse of raisins and other fruits, technically called rape. These tuns are worked in pairs, one being filled with the vinegar from the barrels, and the other tun only three fourths filled. In the latter, the fermentation takes place more rapidly, and the process is rendered more active, alternately, in one or the other tun, by filling up each daily from the other until the process is completed.

Vinegar is often made from cider. The cider is placed in barrels with their bung-holes open. These barrels are exposed during the summer to the heat of the sun. The acetification is completed in the course of about two years. The progress of the fermentation must be watched, and as soon as perfectly formed it should be drawn off into clean barrels.

Without this precaution the acetous fermentation would pass into the putrefactive, and the whole of the vinegar would be spoiled.

Malt Vinegar has a yellowish-red color. The strongest kind, called " Proof Vinegar," contains from four to five per cent, of acetic acid: that of British manufacture usually contains sulphuric acid. The law allows the addition of the one thousandth part of this acid.

Wine Vinegar is nearly one sixth stronger than pure malt vinegar. It is of two sorts, the white and the red, according as it is prepared from white or red wine.

To Distinguish White Wine From Malt Vinegar

Add one ounce of water of ammonia to the same quantity of the vinegar, which, if it is white wine, will produce a purplish muddliness, and a purplish precipitate; and malt vinegar produces either no effect, or a dirty brownish precipitate.