28 MICRO-ORGANISMS AND FERMENTATION. 



If, now, we ask, whether it is advisable to employ any 

 of the various methods mentioned above for the purification 

 of an unknown and impure yeast-mass, the answer must be 

 in the negative ; and this will be the case whether the culture 

 is intended for purely scientific or for industrial purposes, 

 for the danger will always remain of forcing the growth of 

 species other than the desired one. The starting-point being 

 uncertain, it necessarily follows that the result must be so 

 too. In fact, all such methods must now be regarded as 

 antiquated, and will, whenever resorted to, prove utter failures. 

 Nevertheless, they may possibly be used in certain cases 

 of rare occurrence before proceeding to the preparation of 

 a pure culture. In the different branches of the fermenta- 

 tion industry there is but one way that will lead to the 

 goal, namely, the application of the same principles which 

 have for many years been followed in agriculture and horti- 

 culture the selection, l>y means of methodical experiments, of 

 the particular species or type which gives the best results under 

 the existing circumstances, and which is therefore to be sown 

 alone, without admixture of other types. The only possible 

 way of effecting this is, however, by the adoption of the 

 methods discovered by HANSEN, which will come under con- 

 sideration later on. 



(/3) Dilution methods. The second group of methods 

 employed for physiological purposes embraces the dilution 

 methods, or the so-called " fractional cultivation," the 

 principle of which is to dilute the material to such a degree 

 that it is ultimately possible to isolate a single cell. In 

 most of these cultures we can only reckon on their probable 

 purity, whereas for the alcoholic ferments HANSEN has 

 developed the process into an exact method. 



LISTER (1878) was the first to introduce methods of this 

 kind. In order to prepare pure cultures of lactic acid 

 bacteria he first determined microscopically the number of 

 bacteria in a very small drop of sour milk, counting them 

 in several fields of the preparation, and thus calculating 

 their number in the whole preparation. He then estimated 

 the amount of sterilised water it was necessary to add so 

 that after dilution there would be on an average less than 



