i 3 4 WHAT IS THE PURE YEAST OF PASTEUR? 



with certainty whether the flask contains one or several 

 species? If the method is to be called an exact one it must 

 satisfy these demands, but with regard to this neither Pasteur 

 nor Duclaux gives any information. It must, therefore, be 

 candidly stated that for this reason the method is an ex- 

 tremely uncertain one, and that in making use of it we are 

 working in the dark and are really dependent more or less 

 upon chance. 



On examining Pasteur's work more closely (p. 224-228), 

 we find that on the whole he recognised the limits, and that 

 he himself perceived that only a conditional certainty is 

 attainable by the methods which he proposed. In order to 

 obtain a pure culture, he therefore employed not one single 

 method but several, and he states that in different cases it is 

 necessary to employ sometimes one and sometimes another 

 method, in fact, to set to work experimentally ; there is no 

 fixed rule. After describing the different methods, he ex- 

 presses himself as follows : " By means of these different 

 modes of manipulation, separately or combined, it is possible 

 as a ride to obtain the yeast which it is desired to purify, in a 

 very pure condition'' It is thus not a question of pure cul- 

 tures at all, in the sense in which we now employ the term. 

 In describing his experiments with alcoholic ferments he 

 states in several places (e.g. p. 179, in the note on p. 205), 

 that it was not possible to determine whether he had one or 

 several species in each of his flasks. Pasteur tries to attain 

 his object by preparing a series of cultures under conditions 

 as favourable as possible to the particular organism which he 

 wishes to isolate, and at the same time attempts to check the 

 development of such organisms as he desires to remove. By 

 cultivating in this manner, it is naturally only possible to 

 suppress those species which, under the given conditions of 

 nutrition, are unable to withstand competition with the 

 species the development of which has been favoured. There 

 may, however, be a considerable number of other species 



