52 LIFE IN NATUKE. 



But fermentation is excited by the addition of 

 yeast, and proceeds more successfully in proportion 

 to the rapidity with which the yeast cells are 

 developed. Why should this be if the formation 

 of the living cells is only the effect, and not the 

 cause, of fermentation ? 



The intimate connection of growth and decay 

 explains this fact. The yeast excites fermentation 

 because it is itself exceedingly prone to decom- 

 pose ; more prone than the liquid to which it is 

 added. And in decomposing it communicates the 

 impulse of its own change to the matter around 

 it, so disturbing the equilibrium of the elements, 

 and bringing about, in a few hours, chemical changes 

 that would otherwise have occupied a much longer 

 time. And this more active decomposition in the 

 fermenting fluid reacts again upon the cells of the 

 yeast, and produces in them a rapid growth and 

 multiplication. They afford the outlet, as it were, 

 for the force given out by the chemical changes to 

 which they have furnished the stimulus. 



In thus inducing a more vigorous growth by 

 instituting, primarily, a more energetic decay, the 



