In the case of intermixture of alcoholic ferments, we may sometimes manage to effect their separation by taking advantage of their unequal vitalities in different media of cultivation. On December 17th, 1872, we made a powder of commercial Dutch yeast and plaster, as described in Chapter III. § 6. The Dutch yeast was a “high” ferment. On July 25th, 1873, we sowed a portion of this dried mixture in a flask of pure wort. From July 27th patches of bubbles from fermentation were visible on the surface. On August 2nd the fermentation was completed. The yeast, examined under the microscope, was _apparently_ pure, formed of spherical cells of a fine “high” ferment. We poured away the fermented liquid, observing every necessary precaution, and left in the flask almost all the deposit of yeast, and not more than one or two cubic centimetres (about half a tea-spoonful) of beer. On November 15th following the yeast, examined afresh, still seemed pure and still exhibited the form of round cells of “high” yeast, only that they had taken on a very aged aspect, showing a double contour, and filled with granulations collected irregularly about the centre. Such are the precise characteristics of dead cells; nevertheless it was still possible that some living cells yet remained. To assure ourselves of this we took some of the yeast and placed it in a flask of pure wort. On the 19th a little froth from fermentative action appeared on the surface. We then examined the yeast and discovered that it was no longer “high” yeast, but a small ferment of rather irregular appearance, in which the jointed cells of _saccharomyces pastorianus_, as it usually appears after a succession of growths, predominated. It must not be imagined here that what we saw was a transformation of one yeast into another. The phenomena are to be explained much more simply. The Dutch yeast employed being very impure must have contained traces of foreign ferments, especially of _saccharomyces pastorianus_. Reduced to a dry powder on December 17th, 1872, the two or more varieties of cells comprising it had preserved their vitality in consequence of the plaster, and this vitality had continued at all events until July 25th, 1873. Subsequently, when cultivated in wort, they had multiplied in that medium. The _saccharomyces_ had revived like the rest, but its quantity, compared with the high Dutch yeast, was so small that the microscopical observations made on August 2nd, when the flask was decanted, failed to discover its presence. Between August 2nd and November 15th the high yeast must have perished entirely: the cells of _saccharomyces_, on the contrary, still maintained their vitality, and these alone multiplied in the flask of wort impregnated on November 15th. Here we have an example of the separation of alcoholic ferments, through the unequal resistance they sometimes offer to adverse conditions to which they may be subjected. We may also conclude that if we had prepared a quantity of