The purification of ferments may be accomplished by various methods, according as we have to deal with an intermixture of ferments, or to regard as our principal object the expulsion of ferments of disease, such as vibrio germs, lactic ferment, the filamentous ferment of turned beer, _mycoderma aceti_ or _mycoderma vini_. One method of easy application consists in sowing the yeast in water sweetened with 10 per cent. of sugar. This liquid should be first boiled, and preserved in the two-necked flasks which we have so often described. Sweetened water is a very exhaustive medium for ferments, and the organisms mixed with them. A great many cells perish in it, and the chances are that the foreign germs, which are always scarce in comparison with the great number of cells of ferment, may be amongst those which die, or those which become so exhausted that when the yeast, after this treatment, is sown in wort, they disappear, and allow those cells which have remained vigorous enough to develop alone. The addition of a little tartaric acid to the saccharine solution—say, from 1/1000 to 2/1000 part by weight—often facilitates the destruction of certain germs of impurity. _Mycoderma aceti_ and _mycoderma vini_ do not find suitable life-conditions in the sweetened water; they soon disappear if cultivated alternately in sweetened water and wort. In the place of flasks we may make use simply of shallow basins, covered with sheets of glass, such as we have already had occasion to describe, for cultivating yeast in wort after it has been for a longer or shorter time in the sweetened water. The success of these methods of purification is mainly due to the fact that wort is highly aerated, and experience shows that the principal disease-ferments of beer are as much checked in their development by the presence of air as they are favoured by its absence, the inverse of which holds good in the case of alcoholic ferments. So true is this that, working with commercial yeast, which is invariably impure, it would be impossible in our opinion to make beer in closed vessels; and, indeed, as a matter of fact, one has never succeeded in doing this, although the attempt has often been made. To do so requires, much more than in methods actually in use, the employment of pure yeast. There is, therefore, this advantage in cultivating yeasts in shallow basins, that the multiplication of the alcoholic ferments is promoted, and that of most of the disease-ferments is checked. There is an exception, indeed, in the case of mycodermata; but of all disease-ferments these are the most easily got rid of, by repeating our growths before they make their appearance. Notwithstanding this, our two-necked flasks, which also contain much air at first, are to be preferred to the shallow basins, inasmuch as they are a perfect safeguard against the germs floating in the surrounding air, as well as those of the ferment _saccharomyces pastorianus_.