4 HISTORICAL OUTLINE 



alcohol had especially been noted, it was this reaction which Pasteur first selected 

 for experimental study, though he had already made numerous observations on 

 material from the vats of the breweries of Lille. He was probably influenced by 

 the fact that the observations of van Helmholtz (1843) had already indicated that 

 the alcoholic fermentation was due to the yeast itself or to some other organized 

 material. Helmholtz had shown that the substance, whatever it might be, which 

 was responsible for initiating alcoholic fermentation, would not pass through 

 membranes that allowed the passage of organic substances in solution but held 

 back particles in suspension. This experiment, successful with alcoholic ferment- 

 ation, failed with many other ferments and fermentable liquids. Pasteur's mind was 

 naturally addicted to generalization, and his interest lay in the phenomenon of 

 fermentation as a general type of reaction, rather than in one kind of fermentation 

 in particular. It was therefore natural that he should at first neglect the field 

 in which the battle was more evenly balanced between the purely chemical con- 

 ceptions of Liebig, and the biological theories of Cagniard-Latour, Schwann and 

 Helmholtz, and turn to the field in which Liebig's views had never been success- 

 fully attacked. Pasteur's first memoir was published in 1857, and in it he declared 

 the lactic ferment to be a living organism, far smaller than the yeast-cell, but 

 which could be seen under the microscope, could be observed to increase in amount 

 when transferred from one sugar solution to another, and had very decided prefer- 

 ences as regards the character of the medium in which it was allowed to develop ; 

 so that, for instance, by altering the acidity of the medium one could inhibit or 

 accelerate its growth and activity. In this memoir Pasteur laid the first founda- 

 tions of our knowledge of the conditions which must be fulfilled for the cultivation 

 of bacteria. 



These studies on fermentation occupied Pasteur almost continuously from 

 1855 to 1860, and he returned to them again at intervals during later years. He 

 was able to show that the fermentation of various organic fluids was always 

 associated with the presence of living cells, and that different types of fermenta- 

 tion were associated with the presence of microscopic organisms which could be 

 differentiated from one another by their morphology and by their cultural 

 requirements. Thus, at this early stage, the idea of specificity entered into 

 bacteriology. 



It was impossible for Pasteur to pursue these studies without facing the 

 problem of the origin of these minute living organisms, which he regarded as the 

 essential agents of all fermentations. At this time (1859) there were two opposed 

 schools of thought with regard to the genesis of microbial forms of life. One 

 school, deriving their concepts from the great naturalists of antiquity, believed 

 in the spontaneous generation of living things from dead, and especially from 

 decomposing organic matter. It is of little interest to remember the vague terms 

 in which such conceptions were clothed ; but one tendency may be noted, which 

 did not escape the astute mind of Pasteur. The species of animals or plants 

 believed to arise by spontaneous generation were diminishing in number, and the 

 average size of those organisms still included in this category was getting smaller 

 and smaller. In the beginning, the supporters of spontaneous generation were 

 prepared to attribute this mode of origin to relatively large animals. Van Hel- 

 mont, in the sixteenth century, offered a prescription for making mice. It needed 

 the experiments of Redi (1688) to substitute, for the idea that worms were spon- 

 taneously generated in decomposing meat, the truth that these worms were the 



