SYSTEMATICALLY SELECTED YEASTS. 65 



must not be materially altered by the filtration. The Cham- 

 berland filter, according to data to hand, best fulfils these 

 conditions, and the new model which was introduced in 1886 

 was, therefore, selected for the experiments. My assistant, 

 Mr. Poulsen, carried out the experiments under my super- 

 vision, and the following results were obtained : A filter 

 composed of five tubes delivered 10 liters of clear hopped 

 wort (13*5 per cent. Ball.) in 2j hours, when the suction was 

 f atmosphere and the filter tubes were cleaned externally by 

 brushing them every ten minutes. In order to obtain a 

 hectoliter of wort in the same time, 50 tubes would conse- 

 quently be required. Under these circumstances the cleaning 

 would be a somewhat difficult matter, and it might easily 

 happen that first one and then another of so many fragile 

 tubes would get a knock, resulting in larger or smaller 

 cracks. In such a case, the wort entering the fermenting 

 cylinder would be infected, and the operator would be 

 deceived. The most serious fault, however, was that during 

 filtration the wort lost half of the oxygen which it had taken 

 up during aeration on the cooler, and which is necessary for 

 a normal fermentation. 



It is, therefore, not advantageous to employ filtration for 

 sterilising the wort for the fermenting cylinder. In the 

 course of these experiments, however, I perceived that the 

 Chamberland filters would be of great service in preparing 

 different sterile liquids for laboratory experiments. If an 

 absolutely germ-free filtrate is required, the tubes must be 

 sterilised at short intervals ; they must not be kept in action 

 for an indefinite period, as was formerly imagined, for the 

 bacteria pass through the walls after a longer or shorter 

 period. 



We have already learnt that the Chamberland filters yield 

 sterile liquids ; and it is, therefore, also evident that they will 

 yield sterile air. Difficult as it is to obtain sterile liquids 

 by filtration, so it is equally easy to obtain germ-free air. 



F 



