All wines contain one common and identical principle, from which their similar effects are produced; namely, brandy or alcohol. It is especially by the different proportions of brandy contained in wines, that they differ most from one another. When wine is distilled, the alcohol readily separates. The spirit thus obtained is well known under the name of brandy.
All wines contain also a free acid; hence they turn blue tincture of cabbage, red. The acid found in the greatest abundance in grape wines, is tartaric acid. Every wine contains likewise a portion of super-tartrate of potash, and extractive matter, derived from the juice of the grape. These substances deposit slowly in the vessel in which they are kept. To this is owing the improvement of wine from age. Those wines which effervesce or froth, when poured into a glass, contain also carbonic acid, to which their briskness is owing. The peculiar flavour and odour of different kinds of wines probably depend upon the presence of a volatile oil, so small in quantity that it cannot be separated.
EASY METHOD OF ASCERTAINING THE QUANTITY OF BRANDY CONTAINED IN VARIOUS SORTS OF WINE.
The strength of all wines depends upon the quantity of alcohol or brandy which they contain. Mr. Brande, and Gay-Lussac, have proved, by very decisive experiments, that all wines contain brandy or alcohol ready formed. The following is the process discovered by Mr. Brande, for ascertaining the quantity of spirit, or brandy, contained in different sorts of wine.
EXPERIMENT.
Add to eight parts, by measure, of the wine to be examined, one part of a concentrated solution of sub-acetate of lead: a dense insoluble precipitate will ensue; which is a combination of the test liquor with the colouring, extractive, and acid matter of the wine. Shake the mixture for a few minutes, pour the whole upon a filtre, and collect the filtered fluid. It contains the brandy or spirit, and water of the wine, together with a portion of the sub-acetate of lead. Add, in small quantities at a time, to this fluid, warm, dry, and pure sub-carbonate of potash (not salt of tartar, or sub-carbonate of potash of commerce), which has previously been freed from water by heat, till the last portion added remains undissolved. The brandy or spirit contained in the fluid will become separated; for the sub-carbonate of potash abstracts from it the whole of the water with which it was combined; the brandy or spirit of wine forming a distinct stratum, which floats upon the aqueous solution of the alkaline salt. If the experiment be made in a glass tube, from one-half inch to two inches in diameter, and graduated into 100 equal parts, the per centage of spirit, in a given quantity of wine, may be read off by mere inspection. In this manner the strength of any wine may be examined.
Tabular View, exhibiting the Per Centage of Brandy or Alcohol[40] contained in various kinds of Wines, and other fermented Liquors.[41]