The influence of the air on the vital action of the yeast may be proved in ways innumerable. The following is an experiment which we have often carried out with surprising results. A fermentation is going on; we draw off the liquid as rapidly as we please, and pour it back again into the vessel immediately. Within an hour we find a marked increase in the fermentation, evidenced by the liberation of a greater quantity of carbonic acid gas. This experiment may be performed with especial ease if we use the fermenting apparatus that we have described, for, by fitting a gas measurer to the escape tube _a b c d e f g_, the number of litres produced before the drawing off of the liquid may be compared with those obtained after. The least physical change in the running of the fermenting liquid whilst it is being drawn off, modifies the effect in question; such as change in the diameter of the stream, the height from which it falls, its greater or less scattering in falling, all influence it. Again, as might be expected from such results, corresponding modifications take place in the cells of yeast which come under the influence of the air. They become firmer in aspect and outline, their plasma becomes fuller, assumes a younger and more transparent aspect, and the vacuoles disappear. The molecular granulations, too, are less apparent. At a certain focus they disappear; at another they reappear, not as black spots, however, but as brilliant points so small as to be scarcely perceptible. If germination has been suspended it is resumed; in short, everything tends to prove—and having the yeast actually under our eyes we cannot doubt the fact—that the life of the cells is more decided, and the work of nutrition more active after they have been brought into contact with the oxygen of the air, and have absorbed a greater or less quantity of that gas. Under the ordinary conditions of brewing, the atmospheric air is present in very varying quantities, whether introduced by the wort which holds more or less in solution, or by diffusion over the surface of the vessels, so that the same cells of yeast live by turns without air and with air. At first they absorb all the oxygen held in solution, and multiply under the influence of this absorption. Afterwards, when the supply has been exhausted, and various assimilations have resulted from it, they are deprived of it. Their life continues apart from oxygen, and if the vessel were closed, fermentation would be accomplished under these conditions, although more slowly. The vessel being open, a small quantity of air diffuses continuously through the layer of carbonic acid gas on the surface, and supports the vitality of the cells.