The _kefir-grains_ which are added to milk for the preparation of kefir, contain in large numbers a Bacterium (_Dispora caucasica_) in the zooglœa-form, a Yeast-fungus, and _Bacillus lacticus_. Kefir is a somewhat alcoholic sour milk, rich in carbonic acid; it is a beverage manufactured by the inhabitants of the Caucasus, from the milk of cows, goats, or sheep, and is sometimes used as a medicine. In the production of kefir, lactic acid fermentation takes place in one part of the sugar contained in the milk, and alcoholic fermentation in another part, and the casein which had become curdled is partially liquefied (peptonised) by an enzyme of a Zooglœa-bacterium. _Bacillus amylobacter_ (_Bacillus butyricus_), the Butyric-acid-bacterium (Fig. 29), is a very common anaerobic which produces fermentation in sugar and lactic-acid salts, and whose principal product is _butyric acid_. It destroys articles of food and (together with other species) plays a part in the butyric acid fermentation which is necessary in the making of cheese; it is very active wherever portions of plants are decaying, in destroying the cellulose in the cell-walls of herbaceous plants, and is thus useful in the preparation of flax and hemp. The cells are self-motile, generally cylindrical, sometimes united into short rows; endosporous; the spore-forming cells swell, assume very different forms, and show granulose reaction. The germ-tube grows out in the direction of the long axis of the spore. _Bacillus subtilis_, the Hay-bacillus, is developed in all decoctions of hay; a slender, aerobic, self-motile Bacillus; endosporous (aplanospores); the spore-wall ruptures transversely on germination. _Crenothrix kuehniana_ occurs in the springs of many baths, in wells, in water or drain-pipes. [Illustration: FIG. 31.--_Beggiatoa alba_: _a_ from a fluid containing abundance of sulphuretted hydrogen; _b_ after lying 24 hours in a solution devoid of sulphuretted hydrogen; _c_ after lying an additional 48 hours in a solution devoid of sulphuretted hydrogen, by this means the transverse walls and vacuoles have become visible.]