ALCOHOLIC FERMENTS. 123 



his first method a liquid medium was employed, and in his 

 second method a solid medium, for the cultivation ; in both 

 cases the culture was previously diluted as already described 

 (Chapter I., 7. Preparation of the pure culture). 



With the help of the acquired knowledge of the species 

 it was possible to submit these methods to a searching 

 examination, with the result that they proved to be 

 reliable. 



If it is desired to isolate from a mixed growth of different 

 species those which are in an enfeebled condition it is neces- 

 sary, as Hansen points out, to employ the dilution method, 

 using a suitable nutrient fluid, as, for example, wort, the 

 conditions being then favourable for the growth of the 

 organisms in question. 



If, on the other hand, we wish to separate from a mixed 

 growth a species which is in a vigorous state of development, 

 and whose further growth is consequently not dependent upon 

 specially favourable conditions of nutriment, we can attain 

 our object more readily and in a shorter time by the employ- 

 ment of a solid nutrient medium in this case gelatine and 

 wort. It has been proved that the addition of gelatine to 

 wort diminishes its value as a nutritive material for the yeast- 

 fungi. A series of experiments carried out by Holm show in 

 fact that, if at the commencement of a fermentation when 

 the yeast-cells are in their most vigorous state of develop- 

 ment, some of these cells be introduced into wort-gelatine, 

 about 4 per cent, of those sown do not develop ; if, on the 

 other hand, the yeast-cells are taken at the conclusion of a 

 fermentation, when they are enfeebled, about 25 per cent, of 

 them give no colonies in wort-gelatine. 



The advantage of this method, as employed by Hansen, 

 for the study of the budding-fungi is that it makes it possible 

 to directly observe the individual cells under the microscope 

 and to follow their further development, since the gelatine 

 plate is enclosed in a moist chamber (compare page 26, 

 Dilution methods). 



