In addition to these two main products of glucose by fermentation, namely, alcohol and carbonic acid gas, there are glycerine and succinic acid, as well as a lesser proportion of other derivatives, very much akin to alcohol.  Of all these glycerine is by no means unimportant, as it confers a blandness or mellowness upon the wine.  The succinic acid, also, is distinctive for this reason, that it is the source of that characteristic flavour in wine known as “vinosity.”

Besides the water and the glucose, the must likewise contains quite an appreciable amount of those important bodies, the various acids.  These consist of tartaric acid, so frequently met with all through the vegetable world; of malic acid, which is the acid almost distinctive of apples; of tannic acid or “tannin,” and of other acids.  These different acids play an important part in the production of wine; without them, in truth, it would be a mere admixture of spirits and water—­a colourless, flavourless, and insipid product.  By their assistance, however, wine is endowed with the brilliancy it possesses.  And more than this, the action of the alcohol on these acids develops those exquisitely delicate ethers—­the oenanthic and other ethers—­which constitute, in fact, the bouquet of the wine.  At the same time, it has also to be remembered that while these many acids constitute the life and soul, so to speak, of the wine, their very presence is absolutely necessary for the process of vinous fermentation.  That is to say, the active agents of vinous fermentation are only enabled to work perfectly in a liquid which is somewhat acid.

There is an astringent principle, named tannin, which calls for attention in any reference to wine-making.  It is almost the same body—­ not quite—­as the tannin obtained from galls, and so largely employed in tanning.  This vine-tannin, if it may be so termed, does not exist in the juice of the grape, but in the stalk and the skin.  The white wines, in which the juice is almost always freed from the skins and stalks, contain but little tannin; while, on the contrary, most red wines, in which juice, skins, and stalks are all included together in the fermenting-vat, contain a good deal.  Some white wines derive their tannin from the oaken casks which hold the wine; and their colour, in consequence, subsequently deepens.  Other red wines, strange to say, gradually lose their dark colour from a certain action of the tannin.  So that tannin is the cause of some white wines deepening in colour, while it renders other red wines of a lighter colour.  Now, tannin has the effect of preserving albuminous substances, and in this way it may be beneficial in rendering red wines more durable.  But although this may be advisable in wines which are liable to turn, it is certain that excess of tannin is most undesirable.  In fact, the practice of placing the stalks in the fermenting-vat is in many cases, as I have previously stated, an unnecessary proceeding.