_Bacillus Butyricus._ Long and short rods, generally straight, with rounded ends, single or in chains, reproducing themselves both by fission and spores, and sometimes growing out into long threads, actively motile, anaërobic, and liquefying. The spores are widely distributed in nature, and grow readily on fleshy roots, old cheese, etc. The favourable temperature is blood-heat, and on liquid media they produce a pellicle. The resistant spores are irregularly placed in the rod, and may cause considerable variations in morphology. The culture gives off a strong butyric acid odour. It grows most readily at a temperature of about 40° C. Although, according to Pasteur's researches, the butyric acid ferment performs its functions anaërobically, many butyric organisms can act in the presence of oxygen, and yield somewhat different products. All of them, however, ferment most actively at a temperature at or about blood-heat, and the spores are able to withstand boiling for from three to twenty minutes (Fitz). It will be observed that as in lactic acid fermentation so in butyric, the results are not due to one species only. 5. _Ammoniacal Fermentation_ (see under Soil). _Diseases in Beer._ We have seen how a knowledge of fermentation has been compiled by a large number of workers. Spallanzani, Schwann, Pasteur, and Hansen all made epoch-making contributions. In the same way the investigations of diseases in beers and wines were carried out by many observers, and were closely connected with those relating to spontaneous generation and mixed cultures of bacteria in fermentation. These so-called "diseases" are analogous to the taints occurring in milk and due to fermentations. Turning (_tourne_), turbidity, ropiness, bitterness, acidity, mouldiness, are all terms used to describe these diseases. They are chiefly brought about by four agencies:-- 1. Bacteria. 2. Mixed yeasts. 3. "Wild" yeasts. 4. Moulds. To each species of wild yeast there belongs some taint-producing power in the fermentations for which it is responsible. _Saccharomyces ellipsoideus II._ and _S. pastorianus I._, _III._, are such yeasts; they only produce their diseases when introduced at the commencement of the fermentation. _Saccharomyces pastorianus I._ is a low fermentative yeast in elongated cells, producing a _bitter_ taste to beer and an unpleasant odour. It can also produce turbidity. _S. pastorianus III._ produces turbidity, and _S. ellipsoideus II._ has a similar effect. In 1883 Hansen demonstrated that the much-dreaded turbidity and disagreeable tastes and smells in beer may be due to mixture of two yeasts, each of which by itself gives a faultless product.