26 MICRO-ORGANISMS AND FERMENTATION. 



observations, under a moist glass-shade ; thus, an unbroken 

 observation was not attempted, and was not even possible for 

 the larger fungi. It follows from the whole arrangement of 

 the experiment, that absolutely pure cultures were out of the 

 question. As stated above, however, such an investigation 

 may well be carried on with impure material. 



(5) Pure cultures for biological and physiological ex- 

 periments 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 of the whole culture 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. 



(a) Physiological methods. The physiological methods 

 employed by PASTEUR, COHN, and others, start with the funda- 

 mental 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 conditions are 

 favourable. Different liquids have been employed for such 

 cultures from time to time ; as, for instance, alkaline liquids 

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

 yeast-growths from bacteria (lactic, tartaric, hydrofluoric 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 evident that any agency 

 exerted will be hap-hazard, and this does not, properly 

 speaking, constitute a method. In fact, there is always the 

 possibility that the weaker species are not destroyed at all, 

 but merely checked and retarded, so that when the stronger 

 species, after having reached the height of their development, 

 enter into a condition of weakness, other species will have 

 a chance of multiplying. Likewise, there is always the 



