226 STUDIES ON FERMENTATION. 



less liable to injury, less dependent on actual commercial 

 requirements, capable, that is, of withstanding conditions 

 favourable to the development of ferments prejudicial to the 

 soundness of the produce, what we have named ferments of 

 disease. 



In the case of intermixture of alcoholic ferments, we may 

 sometimes manage to effect their separation by taking advan- 

 tage of their unequal vitalities in different media of cultivation. 

 On December 17th, 1872, we made a powder of commercial 

 Dutch yeast and plaster, as described in Chapter III. § 6. The 

 Dutch yeast was a " high " ferment. 



On July 25th, 1873, we sowed a portion of this dried mixture 

 in a flask of pure wort. From July 27th patches of bubbles 

 from fermentation were visible on the surface. 



On August 2ud the fermentation was completed. The 

 yeast, examined under the microscope, was apparently pure, 

 formed of spherical cells of a fine " high " ferment. We 

 poured away the fermented liquid, observing every necessary 

 precaution, and left in the flask almost all the deposit of yeast, 

 and not more than one or two cubic centimetres (about half 

 a tea-spoonful) of beer. 



On ^November 15th following the yeast, examined afresh, 

 still seemed pure and still exhibited the form of round ceUs 

 of *^ high ^' 3^east, only that they had taken on a very aged aspect, 

 showing a double contour, and filled with granulations collected 

 irregularly about the centre. Such are the precise charac- 

 teristics of dead cells ; nevertheless it was still possible that 

 some living cells yet remained. To assure ourselves of this we 

 took some of the yeast and placed it in a flask of pure wort. 

 On the 19th a little froth from fermentative action appeared on 

 the surface. We then examined the yeast and discovered that 

 it was no longer " high " yeast, but a small ferment of rather 

 irregular appearance, in which the jointed cells of saccharomyces 

 pasforianus, as it usually appears after a succession of growths, 

 predominated. It must not be imagined here that what we 

 saw was a transformation of one yeast into another. The 



