MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 27 



possibility that not one but two or more species thrive equally 

 well in the liquid, and, consequently, develop to the same 

 extent. Such, for instance, was the case with brewers' yeast 

 before pure cultures were employed. This yeast often yielded 

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

 are termed, when examined by HANSEN'S method. The 

 method given by PASTEUR for the purification of 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. HANSEN'S investiga- 

 tions 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 in 

 beer, 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 " disease " yeasts, these are apt to develop to such an 

 extent during this treatment that they may eventually 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. 



The use of hydrofluoric acid or its combinations, such as 

 ammonium fluoride, for the purpose of purifying an impure 

 yeast brewers' or distillers' yeast as proposed by EFFRONT, 

 is liable to lead to the same dangers as the use of tartaric 

 acid described above. Methodical experiments made by HOLM 

 and the author have shown that by treating impure yeast 

 according to EFFRONT'S directions, the growth of wild yeast and 

 Mycoderma species is forced more than that of the culture-yeasty 

 they have also shown that such a dangerous species as 

 Bacterium aceti is in many cases not suppressed at all by 

 the treatment in question, but, on the contrary, multiplies 

 more rapidly in presence of hydrofluoric acid or fluorides. 



