220 STUDIES ON FERMENTATION. 



saccharine woi-ts employed should also be exempt from these 

 impurities, as well as the air, which is being constantly renewed 

 at the surface of the liquids. These last conditions may be 

 realized by the adoption of our double-necked flasks, with 

 which a laboratory for research of this kind should be fur- 

 nished, always ready for use, filled with the different kinds 

 of liquids that may be required. 



In general, the inconveniences resulting from the impurity 

 of a yeast employed do not immediately manifest themselves, 

 in consequence of the enormous preponderance of the true 

 yeast, which, in comparison with the foreign germs that con- 

 taminate it, may be so great that microscopical examination 

 fails to reveal even the presence of these latter. Again, it is a 

 well-known fact that the abundance of one growth in a limited 

 medium operates to the prejudice of a less abundant one, 

 inasmuch as the first consumes the uutritive materials at the 

 expense of the second, and more particularly the needful 

 amount of oxygen. It follows, that when a saccharine liquid 

 is impregnated with commercial yeast, nothing but yeast may 

 be detected for a time, and one is led to believe in the purity 

 of the subsequent growth. This, however, supposes that the 

 external conditions, as well as those of the medium of growth, 

 are equally adapted to the life of the yeast and that of those 

 organisms present as impurities ; for if these conditions rather 

 favoured the nutrition of the latter, we should be sure to find 

 their proper developments appearing at an early stage. For 

 example, when the growth of yeast becomes sluggish, we have 

 invariably the development of such after- growths. The prin- 

 cipal germs, having exhausted the saccharine liquid which has 

 fermented under their influence and is no longer adapted for 

 their growth, cease to develop, and have their place taken by 

 ferments of disease, spores of moulds, mycodermata, &c., the 

 growth of which proceeds more or less rapidly, in proportion as 

 the character of the liquid and the surrounding temperature 

 are more or less suited to their growth. 



Here, too, we have an explanation of the rapid change that 



