Wort treated thus will preserve its purity for an indefinite time, notwithstanding its extreme liability to rapid change when exposed to the air, under conditions which cause it to come in contact with the particles of dust that air contains. This also holds good in the case of wine, beef-tea, the must of grapes, and, generally speaking, of all liquids which are subject to putrefaction or fermentation, and which possess the faculty, when their temperature is raised to about 100°C. (212°F.), of destroying the vitality of those microscopic germs that are found in dust. A flask such as we have described (Fig. 3) is all that we require when we have to demonstrate the facts that we have just reviewed, a more detailed account of which may be found in our Memoir published in the _Annales de Chimie et de Physique_, for 1862, under the title _Mémoire sur les corpuscules organisés en suspension dans l’atmosphère. Examen de la doctrine des générations spontanées._ The shape of the flask represented in Fig. 4 only differs from that of the preceding one in having a second little tube attached to the globular part of the flask; this presents great advantages for different objects of study, and it was adopted in the subsequent investigations detailed in this work. This flask will permit us to study without difficulty every separate kind of microscopic organism in the liquid best adapted to it without fear, if we take reasonable precautions, that the subject of our study may become associated with other organisms, the accidental presence of which cannot fail to seriously affect the results of our observations. Let us use one of those flasks in our experiment on yeast, at the same time expressly assuming that the minute germs of yeast are free from all contamination by foreign germs, an object which we shall learn how to realize by a variety of methods in a subsequent chapter. [Illustration: Fig. 4.]