Aside from the crude and often wild notions which have appeared here and there in literature of previous centuries, about the first accurate investigations bearing upon this subject were with reference to the cause of alcoholic fermentation. About the beginning of this century Appert published a monograph upon the Art of Preserving Animal and Vegetable Substances, which consisted in placing them in closely corked or stoppered bottles, and exposing these to the temperature of boiling water. Gay-Lussac, the celebrated chemist, noticed that so soon as these vessels were opened, particularly if much exposed to air, their contents began to at once ferment or putrefy. This led to investigations into the production of alcohol, and the antiseptic effect of pure oxygen-gas; from which he concluded that oxvgen is necessary at the commencement of the process, but not throughout its continuance. Some thirty years later, Schwann, by the use of the microscope, then reasonably developed, discovered in fermenting substances numerous very minute globular bodies, which had the power of reproduction, and which were present in juices or fluids undergoing alcoholic fermentation, but not in others, and which he concluded to be the exciting cause. Schwann also discovered that if, in vessels sealed by Appert's method, lie allowed air which had been previously heated to come in contact with the fluids, no change resulted; from which it was evident that it was something other than the gaseous elements of the air which provoked fermentation. Schwann's investigations were corroborated, in 1843, by Helmholtz. Schwann's results were contested by Liebig, one of the most eminent chemists of his time, who proposed a very different{319} theory, ascribing putrefaction to the absence of oxygen and to the upsetting of molecular arrangements. He believed that non-nitrogenous substances did not spontaneously undergo putrefaction when pure, but they must be brought into contact with some substance already undergoing change, which latter was called a _ferment_, and which converted the oxygen of the air into carbonic acid. According to him, the ferment was some material undergoing decomposition. The next researches on this subject were those of Schroeder and Dusch, in 1854 who studied the question whether filtration of air would prevent the fermentation of boiled fluids to which such filtered air might have access. The material used for filtration was cotton-wool; and they showed that air filtered through it was deprived of the agencies which produce fermentation. Then came Pasteur, who repeated the experiments of his predecessors and elaborated and confirmed them. He also found that it was not necessary to filter the air of its contained particles, but that if it were simply left undisturbed until these had settled by gravity, it might then be brought in contact with putrescible substances without causing any putrefaction.