ALCOHOLIC FERMENTS. 137 



under all conditions, be the employment of a pure cultivation 

 of a selected species of yeast. 



2. The analysis of the yeast in the propagating appara- 

 tus, which must be absolutely pure, is carried out as 

 follows : At the conclusion of fermentation, samples are 

 withdrawn, with every precaution, into Pasteur flasks or into 

 the Hansen flasks employed for sending yeast samples ; from 

 these, small quantities are introduced into flasks containing 

 yeast extract, and these are maintained at a temperature of 

 25 C., the object being to test the yeast for bacteria. The 

 remainder is set aside for the yeast to settle, the beer is 

 decanted, and a sample portion of the sediment is introduced 

 into a sugar solution containing some tartaric acid. After 

 three or four cultivations in such a solution it is further 

 cultivated a few times in beer-wort, and then tested for spore- 

 formation. The smallest traces of wild yeast in the apparatus 

 are brought into a state of vigorous development by this 

 treatment (see Chapter I., 7. Physiological methods). 



(c) The Formation of Films. By the observation of the 

 formation of films, Hansen has found characteristics for the 

 Saccharomycetes in a manner quite different from that given 

 above. A new path for the study of these fungi was thus 

 again opened up, since the statements hitherto made by 

 different authors in this direction are not in accordance with 

 their true behaviour. 



It is a very generally-known phenomenon, that ferment ed 

 liquids become coated with films. It is also well-known that 

 the films formed by the budding fungi Mycoderma cerevisice, 

 Mycoderma vini have especially attracted attention ; and 

 the frequent mention of such films in the literature of the 

 subject led to a result well-known also in other branches of 

 science ; they were spoken of as if well understood for so 

 long that at last the belief in the actual existence of this 

 knowledge became firmly rooted. After Hansen had sub- 

 mitted this question to an experimental investigation, he 

 showed, however, that this view was erroneous. 



