For many years it has been known that sour milk contained bacteria. Pasteur first described the _Bacillus acidi lactici_, which Lister isolated and obtained in pure culture. Hueppe contributed still further to what was known of this bacillus, and pointed out that there were a large number of varieties, rather than one species, to be included under the term _B. acidi lactici_. We have already seen that these bacilli do not as a rule liquefy gelatine, form spores, are non-motile, and are easily killed by heat. When a certain quantity of lactic acid has been formed the fermentation ceases. It will recommence if the liquid be neutralised with carbonate of lime, or pepsine added. Since Pasteur's discovery of a causal bacillus for this fermentation, other investigators have added a number of bacteria to the lactic acid family. Some of these in pure culture have been used in dairy industry to add to the butter a pure sour taste, a more or less aromatic odour, and a higher degree of preservation. (2) _Butyric Acid Fermentation._ This form of fermentation is also one which we have previously considered. Both in lactic and butyric fermentation we must recognise that in the decomposition of milk-sugar there are almost always a number of minor products occurring. Some of the chief of these are gases. Hydrogen, carbonic acid, nitrogen, and methane occur, and cause a characteristic effect which is frequently deleterious to the flavour of the milk and its products. Most of the gas-producing ferments are members of the lactic acid group, and are sometimes classified in a group by themselves. In cheese-making the gases create the pin-holes and air-spaces occasionally seen. (3) _Curdling Fermentations without Acid Production._ Of these there are several, caused by different bacteria. What happens is that the milk coagulates, as we have described, but no acid is produced, the whey being sweet to the taste rather than otherwise. Digestion of casein may or may not take place. We must now mention several fermentations about which little is known. They are designated by terms denoting the outward condition of the milk, without giving any information respecting the real physiological alteration which has occurred. (4) _Bitter Fermentation._ Some bitter conditions of milk are due to irregularity of diet in the cow. Similar changes occur in conjunction with some of the acid fermentations. Weigmann and Conn have, however, shown that there is a specific bitterness in milk due to bacteria which appear to produce no other change. Hueppe suggests that it may be due in part to a proteid decomposition resulting in bitter peptones. There seems to be some evidence for supposing that the bitter bacteria produce very resistant spores, which enable them to withstand treatment under which the lactic acid succumbs.