Yeast is that viscous sort of deposit which takes place in the vats or barrels of must or wort that is undergoing fermentation. This kind of ferment presents for consideration a physical fact of the most extraordinary character. Take a morsel of the substance and put it in sweetened water, in must, or in dough, which always contains a little sugar; after a time, the length of which varies, a few minutes often sufficing, we see these liquids or the dough rise, so to speak. This inflation of the mass, which is due to a liberation of carbonic acid gas, may cause it to overflow the vessels containing it, if their capacity is not considerably greater than the volume of the matters fermenting. It is equally remarkable that these phenomena are natural and spontaneous; that is to say that the must, the wort, and the dough are able to rise, as we have termed it, when left to themselves, without the least addition of foreign substances. The only difference that may occur in these phenomena is a certain amount of retardation, in cases where the yeast does not reach the saccharine matters in a perfectly natural form, inasmuch as it then requires a certain time to get itself together before it can begin to act. It is necessary, indeed, that sugar should be present; for if we abstracted by some means or other from the must or dough all the sugar contained in it, without touching the other constituents, the addition of yeast would produce no gas. Everything would remain quiet until the moment when signs of a more or less advanced putrefaction showed themselves. Yeast is one of the most putrescible of substances, and it is worthy of notice that its alteration is also the consequence of the formation of one or more ferments, very different, however, from that of which we are speaking. As for the nature of yeast, the microscope has taught us what it is. That marvellous instrument, although still in its infancy, enabled Leuwenhoeck, towards the close of the 17th century, to discover that yeast is composed of a mass of cells. In 1835 Cagnard-Latour and Schwann took up Leuwenhoeck’s observations, and by employing a more perfect microscope, discovered that these same cells vegetate and multiply by a process of gemmation. Since then the physical and chemical phenomena already mentioned, such as the raising of the mass, the liberation of carbonic acid gas, and the formation of alcohol, have been announced as acts probably connected with the living processes of a little cellular plant, and subsequent researches have confirmed these views.