66 BACTERIA. 



from these diseases were carefully examined for organisms, 

 and bacteria and monads were carefully described, but in 

 no case could any definite proof be obtained that any of 

 these stood in any causal relation to either of these diseases. 



It was at this stage that Pasteur took up the work initiated 

 by Cagniard-Latour and Schwann, who had first noted the 

 connection between the growth of the yeast organism and 

 fermentation. He applied to other processes of fermentation, 

 such as those of lactic acid, butyric acid, and acetic acid, the 

 same process of experiment and reasoning which they had fol- 

 lowed, and he was able eventually to prove that the organic 

 ferment in each case had specific characteristics, not only as 

 regards its physiological action in setting up a certain definite 

 form of fermentation, but also as regards the special mor- 

 phology and mode of growth of the organisms that were 

 found during and at the end of the process. 



As these experiments of Pasteur's are now classical it 

 may be well briefly to indicate the lines on which he worked. 

 He first carefully observed the nature of the organic material 

 in which certain fermentations took place, studying, both 

 synthetically and analytically, the best medium for his 

 purpose ; he then, by careful microscopical study, determined 

 what organisms developed most rapidly during the special 

 fermentation process. After making an artificial solution of 

 the substance to be fermented, he added a small quantity of 

 albuminoid material, and a trace of the ash of the special 

 yeast that he wished to grow, in order that there might be 

 sufficient of the necessary salts for the nutrition of the 

 organism. This fluid was carefully sterilized by being boiled 

 in flasks, to which only filtered air afterwards had access. 

 To the germ-free solution he added a small quantity of his 

 special yeast, and if he then obtained a characteristic 

 fermentation with the production of the natural special 

 fermentation products, accompanied by rapid growth and 

 multiplication of the organism that he had introduced, he 

 came to the conclusion that this organism was the cause of 

 the special fermentation. 



In 1857 Pasteur described a new yeast (the cells of which 

 were much smaller than the ordinary beer yeast), which gave 

 rise to the formation of lactic acid from sugar, and he pointed 

 out that the nitrogenous material which was necessary for 

 the production of lactic acid was really needed for the 



