In reviewing these ferments we may naturally ask ourselves the question whether the “high” ferments of which we have spoken—the industrial one concerned in the “high” fermentation of breweries, and the other which we have termed _new “high” ferment_—are not aërobian ferments of “low” yeasts. We are inclined to think that the ferment which in the preceding paragraph we termed _new “high” ferment_, may, perhaps, be the aërobian form of the “low” yeast employed by Alsatian and German brewers. We have studied this new “high” ferment side by side with the aërobian ferment of “low” yeast, and the result we have arrived at is, that in appearance and mode of germination, as well as in the flavour and quality of the beers which they produce, they greatly resemble one another. In the last respect, however, we cannot say that the identity is quite absolute, and hence it is with some doubt that we suggest the possible identity of the two ferments. As regards the ordinary “high” yeast of breweries, it may well be supposed, both from its power of rising to the surface during fermentation and from the peculiar smell and flavour of its beer, that we have in it the aërobian ferment of some “low” yeast, as to the identity of which, however, we can say nothing, having no knowledge as to where it is to be found; or, indeed, any certainty that such a yeast actually exists. In writing these lines an idea suggests itself which might be profitably made the subject of serious experimental study. What would be the peculiar properties of the aërobious ferment-form of an aërobian yeast? Certain facts incline us to believe that these forms differ from each other just as a “low” yeast differs from its aërobian ferment. If this were actually the case it would be very interesting to compare the peculiar properties of an indefinite series of aërobian ferments, all derived from a common origin. We find recorded in our laboratory notes that a certain aërobian ferment of the second generation produced a beer different from that produced by the same ferment of the first generation, being possessed of a fragrance so marked that, on entering our laboratory, in which only a few litres of this beer were fermenting, we were at once struck by the powerful odour which it emitted. § VI.—The Purification of Commercial Yeasts.