A process of freezing is likewise employed for the improvement of wine. It results in the removal of much of the cream of tartar, colouring matter, and nitrogenous substances contained, and also causes an increase in the alcoholic strength of the wine, thereby considerably decreasing its tendency to undergo an after-fermentation. The proportions of sugar and acid best adapted to the production of wine of good quality are at least 20 per cent. of the former to not more than 0·5 per cent. of the latter. As these conditions do not always obtain in grape-juice, artificial methods are employed to supply the necessary constituents. Of these, the most rational consists in diluting the must until the amount of acid is reduced to 0·5 per cent., and increasing the sugar to a proportion of 20 per cent. by the addition of glucose. In a somewhat similar process, due to Petiot, the marc is repeatedly mixed with water containing 20 per cent. of sugar, and then subjected to fermentation. In other methods, the removal of the excess of free acid is effected by neutralisation with pulverised marble or neutral potassium tartrate. The use of these agents results in the formation and subsequent separation of insoluble salts--in the latter case, of potassium bitartrate. Another process for the improvement and preservation of natural wine, proposed by Scheele, consists in the addition of glycerine, in a maximum proportion of 3 per cent., after the first fermentation has taken place. R. Kayser[91] has made a very exhaustive investigation of wine-must of different sources, and of the wine prepared therefrom, both in its natural state and after having been subjected to various “processes of improvement.” The following table shows the results obtained from the analysis of Franken must and wine (both natural and “improved”), made from Riessling grapes in 1880:--