On November 27th, 1873, we sowed a trace of this ferment in a flask of wort. From the 29th, with a continuous temperature of 25° C. (77° F.), a considerable deposit of yeast began to form, and the froth of fermentation covered the whole surface of the liquid. We took a little of this deposit for examination; it is represented in Fig. 56. The field is occupied with oval cells of great uniformity. We recognize the aspect of the original yeast (Fig. 43). Here and there, indeed, we come across some cells of larger size, such as those at _a_ and _b_, which is another illustration of the remark that we have just made respecting the forms which revived exhausted cells take on at the commencement of a new germination. The physical aspect of the several aërobian ferments is in general so characteristic that we are often able by simple inspection to distinguish between them as they occur on the surface of liquids. _Saccharomyces pastorianus_ in its aërobian state forms a crown of cells round the sides of the vessel at the surface of the liquid, which crown is broken up by the least agitation of the liquid; its vitality continues for years. The aërobian ferment of “high” yeast appears in the form of small isolated teats on the surface of the fermented liquid. It develops rather sluggishly, and has no great vitality. The aërobian form of “low” yeast develops as a somewhat fragile layer, the least agitation precipitating it to the bottom of the vessel in a cloud of very small irregular flocks, that do not diffuse through the liquid as they fall. With free access to air it retains life for a long time. The aërobian ferment of caseous yeast forms a continuous greasy-looking pellicle, gradually thickening, which breaks up into fragments when shaken. With a supply of air it lives very long, and the pellicle gradually increases in thickness.