We have already stated that the researches detailed in the preceding chapter require for their successful prosecution that the ferments on which we experiment should be absolutely free from germs of other organisms, and we have shown how impossible it would be, if this condition were not complied with, to follow for weeks or months, sometimes even years, the changes which occur in a yeast maintained in contact with air, either in sweetened water or in a liquid which has fermented under its influence. Equally necessary is it that the saccharine worts employed should also be exempt from these impurities, as well as the air, which is being constantly renewed at the surface of the liquids. These last conditions may be realized by the adoption of our double-necked flasks, with which a laboratory for research of this kind should be furnished, always ready for use, filled with the different kinds of liquids that may be required. In general, the inconveniences resulting from the impurity of a yeast employed do not immediately manifest themselves, in consequence of the enormous preponderance of the true yeast, which, in comparison with the foreign germs that contaminate it, may be so great that microscopical examination fails to reveal even the presence of these latter. Again, it is a well-known fact that the abundance of one growth in a limited medium operates to the prejudice of a less abundant one, inasmuch as the first consumes the nutritive materials at the expense of the second, and more particularly the needful amount of oxygen. It follows, that when a saccharine liquid is impregnated with commercial yeast, nothing but yeast may be detected for a time, and one is led to believe in the purity of the subsequent growth. This, however, supposes that the external conditions, as well as those of the medium of growth, are equally adapted to the life of the yeast and that of those organisms present as impurities; for if these conditions rather favoured the nutrition of the latter, we should be sure to find their proper developments appearing at an early stage. For example, when the growth of yeast becomes sluggish, we have invariably the development of such after-growths. The principal germs, having exhausted the saccharine liquid which has fermented under their influence and is no longer adapted for their growth, cease to develop, and have their place taken by ferments of disease, spores of moulds, mycodermata, &c., the growth of which proceeds more or less rapidly, in proportion as the character of the liquid and the surrounding temperature are more or less suited to their growth.