4. Lastly, one characteristic of “high” yeast, which it shares in common with some other ferments, although not with all, and which, from a practical point of view, deserves special mention, is that as fermentation proceeds the yeast rises to the surface of the liquid. Whilst the process of the manufacture of beer by this ferment is going on, the yeast is seen to work out of the bung-holes, flowing over in considerable quantity. The ferment named after the author, as well as “low” yeast, does not possess this property: it remains at the bottom of the vessels. When “high” fermentation takes place in vessels that are not filled, the ferment forms a thick layer, a kind of cap on the surface of the beer. This characteristic may be witnessed even in the fermentation of very small quantities of liquid. In our flasks, in which the volume of fermenting wort does not exceed 100 c.c. or 150 c.c. (about 4 or 5 fluid ounces), we may perceive, as the violence of fermentation subsides, and the head falls, the sides of the vessel covered to a height of from 1 cm. to 2 cm. (about 3/4-in.) above the surface of the liquid, with particles of yeasty matter, in little masses, or in a thin film, raised to that height by the head, and left behind when that fell. “Low” Ferment.—Whilst high yeast performs its functions in the breweries in which it is used at somewhat high temperatures—namely, between 16° C. and 20° C. (60° F. to 68° F.)—“low” yeast is never employed at a higher temperature than 10° C. (50° F.), and it is even thought preferable that it should not be subjected to more than 6° C., 7° C., or 8° C. (43° F. to 46° F.). At these comparatively low temperatures “high” yeast would have no perceptible action, whereas it is at such temperatures that “low” yeast best performs its functions. In our Memoir on alcoholic fermentation, published in 1860, in the _Annales de Chimie et de Physique_, the idea of the identity of the two yeasts was accepted; but we had at that time made no special observations of our own on the subject. Upon closer investigation we are inclined to believe that the two yeasts are quite distinct. We might keep our “high” yeast at the lowest temperatures that it can bear, and repeat our growths under these conditions; or, on the other hand, we might subject the “low” yeast to temperatures higher than those at which it ordinarily grows, without ever succeeding in changing the first into the second or the second into the first, supposing, of course, that each of our yeasts was pure to begin with. If they were intermixed the change in the conditions of development would cause one or the other to preponderate, and incline us to believe that a transformation had really occurred. [Illustration: Fig. 41.]