24 MICRO-ORGANISMS AND FERMENTATION. 



tube of the microscope in order to keep out foreign germs 

 as much as possible. When the development took place in 

 ordinary fluid drops, the preparation was placed, in the interval 

 between two observations, under a moist glass-shade ; thus, 

 an unbroken observation was not attempted, and was not even 

 possible for the larger fungi. Accordingly, in consequence 

 of the whole arrangement of the experiment, absolutely pure 

 cultures are quite out of the question. As stated above, 

 however, such an investigation may very well be carried on 

 with an impure material. 



(6) Pure Cultures for Physiological experiments with 

 mass-cultures. When the object of the pure culture is to 

 employ it for biological or physiological researches, so 

 that a mass-culture of the growth becomes necessary, a 

 direct microscopical control is impossible, and the methods 

 described above cannot be employed. The methods made 

 use of for this purpose may be divided into two groups, 

 namely, the physiological methods and the dilution methods. 

 In the former, liquids are used, in the latter liquids or 

 gelatines. 



() Physiological methods. The physiological methods 

 employed by Pasteur, Colin, and others, start with the 

 fundamental idea, that the various species occurring in a 

 mixture will multiply unequally according to their different 

 natures, when they are cultivated in one and the same nutritive 

 liquid and at the same temperature, so that those species for 

 which the conditions are unfavourable will be gradually 

 suppressed by the one or more species for which the condi- 

 tions are favourable. Different liquids have been employed for 

 such cultures in the course of time ; as, for instance, alkaline 

 liquids for growths of bacteria, acid liquids for the purpose of 

 freeing yeast-growths from bacteria (lactic, tartaric, hydro- 

 fluoric acids, etc.). The weak point of all such methods is, 

 that they start from an unknown material, namely, the 

 impure mixture. Hence, it is impossible to know what 

 results a treatment of this kind will lead to, because it is 



