ALCOHOLIC FERMENTS. 139 



The true Saccharomycetes also form films, which, how- 

 ever, differ somewhat from those mentioned above ; and this 

 is also the case with some of Pasteur's Torula and with 

 Saccharomyces apiculatus. From these observations it is 

 evident that the formation of films is not a peculiarity of cer- 

 tain species, but must be regarded as a general phenome- 

 non common to micro-organisms. 



In the case of the Saccharomycetes this phenomenon 

 generally occurs in the following manner: If cultures in 

 wort are left undisturbed for a shorter or longer time at 

 the ordinary room temperature, small specks of yeast 

 gradually appear on the surface of the liquid after the 

 termination of the primary fermentation ; these can after- 

 wards coalesce to figures of different forms and sizes, to 

 isolated patches, the upper surfaces of which are flat and 

 the under surfaces arched. Finally, they become united to 

 a coherent and generally light greyish-yellow, glutinous 

 film, which may extend to the walls of the glass vessel, 

 forming a complete ring. Such a perfect film-formation 

 only occurs after the primary fermentation is at an end. 

 If the flask be shaken, pieces of the film become detached 

 and sink to the bottom; and in this way a complete layer 

 can gradually collect at the bottom, whilst the film becomes 

 continually renewed and assumes a marbled appearance owing 

 to the younger portions being thin and dark, whilst the older 

 parts are thick and light. 



The conditions under which a film can be formed are a 

 free, still surface, with direct access of air ; and a vigorous 

 film-formation presupposes an abundant supply of air. It 

 follows from this that a far more rapid and vigorous develop- 

 ment will take place in a Chamberland flask, or in an ordinary 

 boiling flask with filter-paper tied over the mouth, than in 

 a Pasteur flask where the admission of air is more limited. 

 The function of film-formation is thus in this respect 

 subject to the same conditions as obtain in the case of 

 endogenous spore-formation. 



