It had been long known, through the observations of a manufacturer of chemical products in Germany, that the impure tartrate of lime of commerce, if contaminated with organic matters and permitted to remain under water in summer, would ferment and yield various products. Pasteur caused the ordinary right-handed tartrate of ammonia to ferment in the following manner:--He took some very pure crystalline salt and dissolved it, adding at the same time to the liquid some albuminoid matter, about one gramme to 100 grammes of the tartrate. The liquid placed in a warm chamber fermented. During the process of fermentation the liquid mass, previously limpid, became gradually turbid, in consequence of the appearance of a small organism which played the part of ferment. Pasteur applied this mode of fermentation to the paratartrate of ammonia. He saw that this salt also fermented, depositing the same organism. All appeared as if the course of things was the same as in the case of the right-handed tartrate. But Pasteur, having had the idea of following the course of the operation with the aid of the polariscope, soon detected a profound difference between the two fermentations. In the case of the paratartrate, the liquid, at first inert, gradually assumed a sensible power of deviation to the left, which augmented by degrees and attained a maximum. The fermentation was then suspended; there was no longer any of the right-handed acid in the liquid, which, when evaporated and mixed with its own volume of alcohol, immediately furnished a beautiful crystallisation of left-handed tartrate of ammonia. From that moment a great new fact was established--namely, that the molecular dissymmetry proper to organic matters intervened in a phenomenon of the physiological order, and did so as a modifier of chemical affinity. The kind of dissymmetry proper to the molecular arrangement of the left-handed tartaric acid was, no doubt, the sole cause of the difference between this acid and the right-handed acid, in regard to the fermentation produced by a microscopic fungus. We shall see later on that organised ferments are almost always microscopic vegetables, which embrace in their constitution cellulose, albumen, &c., identical with these same substances taken from the higher class of vegetables and equally dissymmetric. We can thus understand, that for the nutrition of the ferment and the formation of its principles the chemical changes are more easy with one of the two tartaric acids than with the other.