214 MICRO-ORGANISMS AND FERMENTATION. 



trace of the yeast is transferred to the cotton-wool, and 

 the flask is again closed in the flame with the asbestos 

 stopper, which is then coated over with sealing-wax. When 

 the culture is to be used, the flask is again connected with 

 a Pasteur flask containing wort, and the yeast is rinsed into 

 the latter. 



This method has proved especially valuable for sending 

 absolutely pure yeast to tropical countries. In many cases 

 it would otherwise have been impossible to send pure 

 cultivations to Australia, South America, and the most 

 distant countries of Asia. 1 



It is a fact of the greatest importance, that even after 

 the lapse of years, the perfectly identical yeast once selected 

 can always be had again, a sample of the pure culture 

 being preserved in the laboratory in a 10 per cent, solution 

 of cane-sugar (page 20). In such a solution, the cultivated 

 yeasts can be kept alive and unchanged in their properties 

 for years. 



With regard to the preservation of micro-organisms on 

 solid substrata, Percy Frankland found that bacterial fer- 

 ments sometimes completely lose their fermentative power 

 after repeated cultivation in solid media. In some cases 

 the fermentative power disappeared after a single plate- 

 cultivation. 



The absolutely pure and systematically selected races of 

 yeast, prepared in large quantities for industrial purposes, 

 are now Hansen having made his first experiment in 1883 



1 The use of sterilised filter-paper for sending yeast samples is for 

 quite a different object. This method is used either for sending an 

 impure brewery-yeast to a laboratory in order that a pure culture may 

 be prepared from it, or a sample of a pure culture may be conveniently 

 sent in this manner, enclosed in an envelope ; it is clear, however, that a 

 sample sent in this way can no longer be depended on as absolutely pure, 

 and the sample can therefore only serve as material for a new pure 

 cultivation. 



