MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 25 



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 weak- 

 ness, other species will get a chance of multiplying. Likewise, 

 there is always the possibility that not one but two or more 

 species thrive equally well in the liquid, and, consequently, 

 develop to the same extent. If we examine, for instance, 

 common brewers' yeast, we may often separate several 

 typically different species of " culture-yeast," as they are 

 termed, from the same yeast-mass by means of Hansen's 

 method. The method given by Pasteur for the purification 

 of a brewers' yeast may be mentioned as a marked illustration 

 of the dangers connected with the physiological method of 

 treatment. The impure yeast-mass is introduced into a cane- 

 sugar solution to which a small amount of tartaric acid has 

 been added. The object of the method is to free the yeast from 

 any disease germs with which it may be infected. Hanserfs 

 investigations have, however, proved that, even if the bacteria 

 are suppressed or checked by this treatment, the so-called 

 wild yeasts, and among them those productive of diseases, 

 will develop abundantly, and in many cases the culture-yeast 

 becomes totally suppressed instead of being purified. Even 

 if there is primarily only a trace of the wild yeasts, or yeasts 

 of disease, these are apt to develop to such an extent through 

 this treatment that finally they may form the chief portion 

 of the yeast-mass. Thus, this unmethodical treatment of the 

 unknown material has led to an exactly opposite result to 

 that intended. Even when the yeast-mass consists entirely 

 of the so-called wild yeasts, it is not possible by this process 

 of Pasteur's to prepare with certainty a pure culture of a 

 definite species. 1 



1 According to Effronfs method, hydrofluoric acid is used as an anti- 

 septic in distilleries. But the use of this material for the purification of 



