Van Laer has described a saccharobacillus which produces lactic acid amongst other products, and brings about a characteristic disease in beer, named _tourne_. The liquid gradually loses its brightness and assumes a bad odour and disagreeable taste. The bacillus is a facultative anaërobe. A number of workers have separated organisms, having a lactic acid effect, which diverge considerably from the orthodox type of lactic acid bacillus. This is but further evidence of a fact to which reference has been made: that nomenclature restricted to one individual has now become adapted to a family. 4. _Butyric Acid Fermentation._ Cause, Bacillus butyricus and B. amylobacter; medium, milk, butter, sugar and starch solutions, glycerine; result, butyric acid. When sugars are broken down by the _Bacillus acidi lactici_ the lactic acid resulting may, under the influence of the butyric ferment, become converted into butyric acid, carbonic acid, and hydrogen. Neither butyric acid nor lactic acid is as commonly used as alcohol or vinegar. Both, like vinegar, can be manufactured chemically, but this is rarely practised. Butyric acid is a common ingredient in old milk and butter, and its production by bacteria is historically one of the first bacterial fermentations understood. Moreover, in its investigation Pasteur first brought to light the fact that certain organisms acted only in the absence of oxygen. In studying a drop of butyric fermenting fluid, it was observed that the organisms at the edge of the drop were motionless and apparently dead, whilst in the central portion of the drop the bacilli were executing those active movements which are characteristic of their vitality. To Pasteur's mind this at once suggested what he was able later to demonstrate, namely, that these bacilli were paralysed by contact with oxygen. When he passed a stream of air through a flask containing a liquid in butyric fermentation, he observed the process slacken and eventually cease. So were discovered the _anaërobic_ micro-organisms. The aërobic ferments give rise to oxidation of certain products of decomposition; the anaërobic organisms, on the other hand, only commence to grow when the aërobic have used up all the available oxygen. Thus in such fermentations certain bodies (carbohydrates, fatty acids, etc.) undergo decomposition, and by oxidation become carbonic acid gas, and the remainder is left as a "reduced" product of the whole process. Hence sometimes this is termed fermentation by reduction. The chemical formula of this butyric reaction may be expressed thus:-- C_{6}H_{12}O_{6} (by simple decomposition) = 2 C_{3}H_{6}O_{3}. Glucose, Lactic acid. which is followed by the fermentation of the lactic acid:-- 2 C_{3}H_{6}O_{3} = C_{4}H_{8}O_{2} + 2 CO_{2} + 2 H_{2}. Lactic acid. Butyric acid. Carbonic Free hydrogen. acid gas. [Illustration: B. BUTYRICUS]