We shall conclude this paragraph with a remark that has doubtless presented itself to the minds of our readers, which is, that it would be impossible to carry out the experiments we have described if we could not make sure of dealing with pure ferments, or, at least, with mixtures the components of which are sufficiently well known for us to assign to each the effect produced by it in the total phenomena observed. It would be extremely difficult to continue growths of yeast-deposit in sweetened water or in a moist atmosphere if the little plant were mixed with spores of other fungoid growths, a variety of ferment-forms, and germs of bacteria, vibrios, or infusoria in general. All these foreign organisms would tend to develop just in proportion as the conditions of the media were more or less favourable to their growth, and, in a very few days, our flasks would be filled with swarms of beings which, in most cases, would entirely conceal the facts relating to those forms, the separate study of which it was our object to follow out. We shall have occasion, therefore, to examine, in a subsequent paragraph, the preparation of ferments in a state of purity. At present we may state that yeast, which in its ordinary condition is a mass of cells so liable to change that its preservation in a moist state is impossible, manifesting in the course of a few days during the winter, and in twenty-four hours during the heat of summer, all the signs of incipient putrefaction, thereby losing its distinctive characteristics, is nevertheless capable, when pure, of enduring the highest atmospheric temperatures for whole years without showing the least signs of putrid change or contamination with any other microscopic organisms, and without the cells losing their power of reproduction. In the presence of facts like these, the theory of spontaneous generation must seem chimerical. The hypothesis of the possible transformation of yeast into _penicillium glaucum_, bacteria, and vibrios, or conversely, which the theories of Turpin, H. Hoffmann, Berkeley, Trécul, Hallier, and Béchamp involve, is equally refuted by these facts. § II.—On “Spontaneous” Ferment.