STUDIES ON FERMENTATIOl'f. 1C9 



represented in the left-hand half, in a medium peculiarly 

 adapted to their vitality, and in the presence of much air. As 

 regards the length and size of filaments and cells, there is 

 little appreciable difference between the two sides. The 

 principal difference consists in the relative freshness and the 

 budding going on in individuals in the right-hand half. 



There is a simple means of transforming the small, disjointed 

 forms of the yeast as it occurs in a deposit, at the end of a 

 fermentation, back into the long, tubular, pear-shaped forms 

 peculiar to the germination of the germ-cells, which exist 

 amongst the particles of dust spread over the surface of fruits. 

 Plate X. illustrates the result of the process. For this purpose 

 we must eflfect as complete an exhaustion as possible of the 

 ferment saccharomyces pastorianus, by leaving it to itself for a 

 very long time, without aliment, in contact with pure air, in a 

 damp state ; or, better still, in presence of sweetened water. 

 We cultivate some yeast in wort, in one of our two-necked 

 flasks, and then carefully decanting the fermented liquid 

 through the right-hand neck, leave the deposit of yeast on the 

 sides of the flask. The glass stopper which closes the india- 

 rubber tube must be replaced, and the moist yeast be left thus, 

 in contact with pure air. The cells will steadily continue their 

 activity, and so gradually age, without meanwhile losing their 

 vitality. We use the word age, as we have already observed, 

 because the period of rejuvenescence in the case of such a yeast 

 is so much the slower the longer the plant has remained in 

 that state. 



Under these conditions the yeast rarely dies. It becomes 

 attenuated and shrivelled but still preserves its vitality, that 

 is, the power of reproducing itself after a lapse of several 

 months or even several years. In the end, however, it dies, 

 a fact which is proved by the cells, when sown in a nutri- 

 tive medium, remaining inert. 



To exhaust yeast, without destroying it, sweetened water is 

 preferable. Having decanted the beer, we substitute in its 

 place water sweetened with 10 per cent, of pure sugar. By 



