IV. 



YEAST. 



IT has been known, from time immemorial, that the 

 sweet liquids which may be obtained by expressing the 

 juices of the fruits and stems of various plants, or by 

 steeping malted barley in hot water, or by mixing honey 

 with water are liable to undergo a series of very singu- 

 lar changes, if freely exposed to the air and left to them- 

 selves, in warm weather. However clear and pellucid 

 the liquid may have been when first prepared, however 

 carefully it may have been freed, by straining and filtra- 

 tion, from even the finest visible impurities, it will not 

 remain clear. After a time it will become cloudy and 

 turbid ; little bubbles will be seen rising to the surface, 

 and thei" abundance will increase until the liquid hisses 

 as if it were simmering on the fire. By degrees, some of 

 the solid particles which produce the turbidity of the 

 liquid collect at its surface into a scum, which is blown 

 up by the emerging air-bubbles into a thick, foamy froth. 

 Another moiety sinks to the bottom, and accumulates as 

 a muddy sediment, or "lees." 



When this action has continued, with more or less 

 violence, for a certain time, it gradually moderates. The 

 evolution of bubbles slackens, and finally comes to an 



