142 WHAT IS THE PURE YEAST OF PASTEUR f 



us in reference to this point ; as was pointed out above, we 

 have no indication by means of which we can decide whether 

 this result has been attained or not, and if we carry the 

 treatment further there is the danger that all life will perish. 

 In short, this method of working is and remains one depen- 

 dent upon chance, and it can never become an exact method. 

 The chief difficulty in the employment of the physiological 

 method for the object in view is, as already stated, that we 

 do not know beforehand how the species with which we are 

 working will behave in respect to it ; and even if we make 

 preliminary trials, the same result would by no means be 

 attained under all conditions. In this case, the individual 

 characters of the cells of the species will play a part, and it 

 is highly probable that when a species is exposed to the 

 treatment mentioned for a number of generations, it becomes 

 influenced in such a way that it gradually becomes better 

 able to struggle on under the unfavourable conditions of 

 nourishment. This constitutes a further objection to the 

 employment of the physiological methods for the purpose 

 mentioned. 



The only certain way under all circumstances by which 

 we can obtain a pure culture of a micro-organism, whatever 

 physiological and morphological properties it may possess, 

 is to sow a single cell in a sterilised nutrient medium. 



Experiment V. This and the following experiment were specially 

 undertaken in order to test the method described by Velten in the lecture 

 mentioned above, and which he employs for the purification of brewers' 

 yeast in accordance with Pasteur's directions. The liquid in this case 

 was an aqueous solution, containing ten per cent, of cane sugar and four 

 per cent, of tartaric acid. The experiments were made with cultures 

 consisting of young, vigorous cells, which were obtained by twenty- 

 four hours' cultivation in wort at 26 C, and each species was introduced 

 in equal amount into the flasks. 



In A, Sacch. cerevisia /., Sacch. Pastorianus /., Sacch. Pas tori- 

 anus III. 



In B, Carlsberg bottom yeast No. i, Carlsberg bottom yeast No. 2, 

 Sacch. Pastorianus /., Sacch. Pastorianus III. 



