PASTEUKS EXPERIMENTS 3 



they agree in way of growth as well as in structure. In 

 1837 the important discovery was made by Schwann that 

 the phenomenon of alcoholic fermentation was connected 

 with the presence and life of the yeast-plant, and that 

 putrefaction was due ' to something in the air which heat 

 was able to destroy.' 



Messrs. Schroder and Dusch in 1854 introduced the use 

 of cotton-wool for filtering air to free it from micro-organ- 

 isms, and for plugging apparatus. But it remained for 

 Pasteur to make the greatest advance in this department, 

 by making pure cultures of various organisms, thus render- 

 ing an accurate study of them for the first time possible. 



Pasteur's first experiments were devoted to the study of 

 the yeasts, and the part they play in the phenomena of 

 fermentation. As Pasteur's classical experiments laid the 

 foundations of the modern study of bacteriology, it may be 

 well to describe the general 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 the 

 purpose, and then by careful microscopical study deter- 

 mined what organisms developed most rapidly during the 

 fermentation process. After making a solution of the 

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

 albuminous material and a trace of the ash of the yeast 

 under examination, so that there should bfe a sufficient 

 quantity of the necessary mineral constituents present. 

 The medium was then carefully sterilised by being boiled 

 in flasks to which only filtered air had access. To the 

 germ-free solution he added a small trace of the special 

 yeast which he wished to examine. By this means, after 

 growing the organism through two or three generations, he 

 obtained pure cultures. Pasteur also employed the ' dilu- 

 tion,' or ' fractional,' method of cultivation. A drop of the 



