After having passed the flame of a spirit-lamp quickly over the india-rubber tube, the glass stopper, the curved tube, and even over the fingers of the operator, we may withdraw the glass stopper, and introduce the pure yeast by means of a glass pipette that has been previously heated. This yeast is kept in a vessel also free from the dust floating in the air. However few globules of yeast the glass tube may take up, it is sure to introduce a hundred or a thousand times more than is necessary for the impregnation of the liquid. The glass stopper must then be replaced immediately, after having been again quickly passed through the flame. In transferring our yeast from the vessel containing it to the flask, by means of a glass pipette, it is exposed to another cause of impurity, since we cannot avoid bringing it in contact with the common air. If this risk frequently troubled us in our experiments, we might banish it or minimize it by some new arrangement; but this is unnecessary. We have suffered no inconvenience from this cause, as there does not exist in the atmosphere anything like a continuous supply of that from which the so-called _spontaneous_ generation arises, as was erroneously believed to be the case before the publication of our Memoir in 1862, to which we have already alluded. The following are the results of the experiments conducted in the manner just described. The yeast which we sowed, in ever so small a quantity, seemed to acquire vigour, to bud, and to multiply. Soon, that is to say in the course of twenty-four or forty-eight hours or longer, according to the temperature, and, more especially, the degree of vitality in the globules, we found that the sides of our flasks were covered with a white yeasty deposit, and noticed on the surface of the liquid a fine froth, which at first appeared like little islets formed by groups of bubbles so minute that they would have been imperceptible had they not been joined together. These patches increased in size, and gradually attached themselves to each other, finally forming a thick froth. In the course of two or three days this froth fell, the fermentation proceeded less rapidly, and then ceased completely. The beer was finished. This beer might be preserved for an indefinite period in the flask without undergoing any change. The external air passes freely into and out of the flask, as the pressure of the atmosphere and the temperature vary, and the beer in the course of time becomes flat; it acquires age in much the same manner as wine does, but it never contracts any taste of disease, it never becomes sour, or sharp, or bitter, or putrid; it does not even become covered with _mycoderma vini_ as is usually the case with all beer exposed to the common air in the course of trade.