Pasteur has studied experimentally that which takes place when, without otherwise changing the conditions of these phenomena, the arrangement is so modified as to permit the introduction of the free oxygen of the atmosphere. It sufficed for this purpose to provoke a fermentation of the must of beer, or the must of grapes, upon shallow glass dishes presenting a large surface, or in a flat-bottomed wooden trough with sides a few centimeters in height, instead of in deep vats as before. In these new conditions the fermentation manifests an activity even more extraordinary than it did in the deep vats. The life of the ferment is itself singularly enhanced, but the proportion of the weight of the decomposed sugar to that of the yeast formed is absolutely different in the two cases. While, for example, in the deep vats, a kilogram of ferment sometimes decomposes seventy, eighty, one hundred, or even one hundred and fifty kilograms of sugar, in the shallow troughs one kilogram of the ferment will be found to correspond to only five or six kilograms of decomposed sugar. These proportions between the weight of the sugar which ferments and the weight of the ferment produced, constitute the measure of what one might call the ferment's character--of that character which distinguishes its mode of life from that of all other existences, great or small, in which the weight of the organising matter and the assimilated alimentary matter are about equal. In other words, the more free oxygen the yeast ferment consumes, the less is its power as a ferment. Such is the case in the shallow troughs where the extended surface is exposed to the contact of the oxygen of the air. The more, on the contrary, the life of the ferment is carried on without the presence of free oxygen, the greater is its power of decomposing and of fermenting the saccharine matter. This is the case in deep casks. The intimate co-relation then between life without air and fermentation appears complete. The unexpected light which these facts threw upon the cause of the phenomena of fermentation made a forcible impression upon all thinking minds. 'In these infinitely small organisms,' M. Dumas said one day to M. Pasteur before the Academy of Sciences, 'you have discovered a third kingdom--the kingdom to which those organisms belong which, with all the prerogatives of animal life, do not require air for their existence, and which find the heat that is necessary for them in the chemical decompositions which they set up around them.' The work of Pasteur, demonstrating that fermentation was always dependent on the life of a microscopic organism, continued without interruption. One of the most remarkable of his researches is that which relates to the fermentation of the tartrate of lime. The demonstration of life and of fermentation without free oxygen is in this paper carried to the utmost limits of experimental rigour and precision. III.