In wort-gelatine _Bacterium pasteurianum_ develops round colonies with a smooth or wavy border, whilst _B. aceti_ has a tendency towards stellate arrangement. Spores have not been observed, and from a morphological point of view the two species behave alike. Neither produces any turbidity in the liquid containing them. In order to flourish, _B. aceti_ requires a temperature of about 33° C. and a plentiful supply of oxygen. In a cool store or cellar there is, therefore, nothing to fear from _B. aceti_. Frankland has isolated a _Bacillus ethaceticus_, which is a fermentative organism producing ethyl-alcohol and acetic acid. By oxidation the ethyl-alcohol may be converted into acetic acid. 3. _Lactic Acid Fermentation._ Cause, Bacillus acidi lactici; medium, milk-sugar, cane-sugar, glucose, dextrose, etc.; result, lactic acid. The process set up by the lactic ferment is simply a decomposition, an exact division of one molecule of sugar into two molecules of lactic acid, there being neither oxidation nor hydration. The conditions under which the ferment acts are very similar to those we have already considered. There is frequently carbonic acid gas formed; there is a cessation of fermentation when the medium becomes too acid; there is the same method of starting the process by inoculation of sour milk or cheese or any substance containing the specific bacillus. It is probable that such inoculated matter will contain a mixture of micro-organisms, but if the lactic bacillus is present, it will grow so vigorously and abundantly that the fermentation will be readily set up. [Illustration: B. ACIDI LACTICI] _The Bacillus Acidi Lactici._ Rods about 2 µ long and 4 µ wide, occurring singly or in chains and threads. It is non-motile. Spore formation is present, the spores appearing irregularly or at one end of the rod. On the surface of gelatine a delicate growth appears along the track of the needle, with round colonies appearing at the edges of the growth. It does not liquefy gelatine. It grows best at blood-heat; but much above that it fails to produce its fermentation, and it ceases to grow under 10° C. It inverts milk-sugar and changes it to dextrose, from which it then produces lactic acid. Sugars do, however, differ considerably in the degrees to which they respond to the influence of the lactic ferment, and some which are readily changed by the alcoholic ferment are untouched by the _Bacillus acidi lactici_. It will be necessary to refer again to this micro-organism when we come to speak of milk and other dairy products.