140 MICRO-ORGANISMS AND FERMENTATION. 



Simultaneously with the formation of a film, a decolor- 

 ation of the wort takes place, the latter becoming of a pale 

 yellow colour. This reaction takes place most quickly at 

 the higher temperatures, and occurs most markedly with 

 those species which give rise to the most vigorous film- 

 formation. 



The preliminary cultivation of the cells is the same as 

 that previously described (page 124). The liquid is removed 

 from the growth obtained, and fresh sterilised wort is added ; 

 the mixture of yeast and wort is agitated, and a drop is 

 transferred with the usual precautions to an ordinary flask 

 of about 150 c.cm. capacity, half filled with wort, and a 

 piece of filter-paper is then tied over its mouth. Hansen 

 exposed flasks treated in this way to different temperatures, 

 and determined : 



1. The limits of temperature for the formation of films; 



2. The approximate length of time required for their 

 formation at different temperatures ; and 



3. The microscopic appearance of the growths at different 

 temperatures. 



The main point in these investigations of the six species 

 previously mentioned is the microscopic appearance of the 

 films of these species, formed at the same temperature ; and 

 here again, when regarded from a different point of view 

 to that considered in the last section, we have a complete 

 investigation of the relation between the external interfering 

 factors and the forms, which proves that we have to do with 

 so many perfectly distinct types or species. 



The examination of the films was made, except when 

 otherwise stated, when they were so far developed that they 

 could just be seen with the naked eye. 



A glance at the illustrations representing these film- 

 growths (see page 161 and following pages) will show that 

 their general character is usually different from that of the 

 sedimentary forms. For instance, the sedimentary form of 

 Sacch. cerevisice I. is egg-shaped or spherical, whilst in the 



