In 1836 Chevreuil and Pasteur showed that putrefaction did not occur in meat protected from contamination. In 1837 Caignard-Latour, in France, and Schwann, in Germany, independently showed that alcoholic fermentation in beer and wine is due to the growth of a microscopic plant, the yeast, in the fermenting wort. C. J. Fuchs described the organism which is commonly called the “blue milk bacillus” in 1841 and conjectured that the souring of milk was probably bacterial in origin. It remained for Pasteur to prove this in 1857. During the following six or seven years Pasteur also proved that acetic acid fermentation, as in vinegar making, butyric acid fermentation (odor of rancid butter and old cheese) and the ammoniacal fermentation of urea, so noticeable around stables, were each due to different species of bacteria. Pasteur also, during the progress of this work, discovered the class of organisms which can grow in the absence of free oxygen--the anaërobic bacteria. There is no question that Pasteur from 1857 on did more to lay the foundations of the science of bacteriology than any other one man. Influenced by Pasteur’s work von Hesseling, in 1866, stated his belief that the process of cheese ripening, like the souring of milk, was associated with the growth of fungi, and Martin also, in 1867, stated that cheese ripening was a process which was akin to alcoholic, lactic and butyric fermentations. Kette, in 1869, asserted the probability of Pasteur’s researches furnishing a scientific basis for many processes of change in the soil. In 1873 Schlösing and Müntz showed that nitrification must be due to the action of microörganisms, though the discovery of the particular ones remained for Winogradsky in 1889. Thus the belief that fermentation and putrefaction are due to microörganisms was as well established by the early eighties of the last century as that similar organisms are the causes of infectious diseases. STUDY OF FORMS.