Tormented, however, by the idea that, notwithstanding all these facts, the reasonings of Liebig might still find some credit, Pasteur worked earnestly to discover new facts capable of demonstrating that Liebig's theory was absolutely false. He made two crucial experiments, the one relating to the yeast of beer, or of alcohol, and the other relating to the lactic ferment. He introduced into a pure solution of sugar a small quantity of crystallisable salt of ammonia, then some phosphates of potash and magnesia, and he sowed in this medium an imponderable quantity, if we may so express it, of fresh cells of yeast. The cells thus sown multiplied, and the sugar fermented. In other words, the phosphorus, the potassium, the magnesium of the mineral salts, united to form the substances which compose the ferment. By this experiment, so simple and yet so demonstrative, the power of the organisation of the ferment was once for all established. The contact theory of Berzelius had no longer any meaning, since it was evident that the fermentable matter here furnished to the ferment one of its essential elements, namely, carbon. Liebig's theory of communicated molecular motion, originating in a nitrogenous albuminoid substance, had no better claim, since such substances had been discarded. The whole process took place between the sugar and a ferment germ which owed its life and development to nutritive matters, the most important of which was the fermentable substance. Fermentation, in short, was simply a phenomenon of nutrition. The ferment augmented in weight, feeding upon the sugar, and its vitality was such that it contrived to build up the complex materials of its own organisation by means of sugar and purely mineral elements. In a second experiment, Pasteur demonstrated that, notwithstanding their smallness and the possibility of confounding them with the amorphous granules of caseine and gluten, the little particles of lactic ferment were indeed alive, and that they, and they only, were the cause of lactic fermentation. He mixed with some water, sweetened with sugar, a small quantity of a salt of ammonia, some alkaline and earthy phosphates, and some pure carbonate of lime obtained by precipitation. At the end of twenty-four hours the liquid began to get turbid and to give off gas. The fermentation continued for some days. The ammonia disappeared, leaving a deposit of phosphates and calcareous salt. Some lactate of lime was formed, and at the same time one could notice the deposition of the little lactic ferment. The germs of the lactic ferment had, in this case, been derived from particles of dust adhering to the substances themselves, of which the mixtures were made, or to the vessels used, or from the surrounding air. The chapter on spontaneous generation will render this clear.