It being necessary for the conduct of our experiments to preserve our yeast in a state of purity for an indefinite period, often for a great length of time, in contact with pure air, we discovered that yeast was possessed of extraordinary vitality, and that it rarely perished completely throughout, inasmuch as we could almost invariably cause it to revive by bringing it into contact with fresh, fermentable liquid. This revival of the yeast—and it is to this point that we are most anxious to direct the attention of our readers—is effected from two distinct sources:— 1. By those cells of yeast which have not perished. 2. By cells of new formation. We may give an example to explain this more clearly. In one of our two-necked flasks we cause some pure wort to ferment by employing yeast also in a state of purity. Fermentation completed, we leave the liquid to itself, not touching the flask again. The fermented liquor covers a deposit of yeast, apparently inert, and no trace of _mycoderma vini_ makes its appearance on the surface of the liquid. Let us suppose that we go on daily for a considerable time introducing a little of the yeast from this flask to a different flask of wort: the fresh flasks will begin to ferment. The only appreciable difference which these successive flasks will present, their impregnation having been effected at intervals of twenty-four hours, will be that, _ceteris paribus_, fermentation in them will be more and more slow in making its appearance. This difference, as we have already explained, will be due to the fact that the yeast in the first flask will, in the course of time, undergo, in each of its cells, a process which we cannot better describe than as a progressive _senescence_. The cells gradually become filled with amorphous granulations, their interior becomes yellow, and the protoplasm collects, either at the centre or near the borders; in short, the vitality of the yeast becomes feeble. When, however, it is taken out of the liquid in which it has fermented and introduced into a fresh saccharine wort, it gradually resumes its transparency, and then begins to germinate. These effects are the less rapidly brought about the longer the cells remain exhausting themselves in the first fermented liquid. They might be left in that liquid for such a length of time that they would eventually perish, a fact which would manifest itself in their absolute sterility and quiescence when sown in a fresh medium. In general, however, matters are not carried far enough for this to take place, and the yeast, preserved in a state of purity in its fermented liquid, retains the capacity of revival, which may then go on indefinitely. As a matter of observation, the cells of yeast, after causing the liquid to ferment, instead of remaining inactive, and so by living at their own expense gradually passing into a state of exhaustion, begin to bud again; at least this is true of many of them.