YEAST 



I HAVE selected to-night the particular subject of Yeast for 

 two reasons or, rather, I should say for three. In the 

 first place, because it is one of the simplest and the most 

 familiar objects with which we are acquainted. In the 

 second place, because the facts and phenomena which I 

 have to describe are so simple that it is possible to put 

 them before you without the help of any of those pictures 

 or diagrams which are needed when matters are more 

 complicated, and which, if I had to refer to them here, would 

 involve the necessity of my turning away from you now 

 and then, and thereby increasing very largely my difficulty 

 (already sufficiently great) in making myself heard. And 

 thirdly, I have chosen this subject because I know ol KO 

 familiar subs uance forming part of our every-day knowledge 

 and experience, the examination of which, with a little care, 

 tends to open up such very considerable issues as does this 

 substance yeast. 



In the first place, I should like to call your attention to a 

 fact with which the whole of you are, to begin with, perfectly 

 acquainted, I mean the fact that any liquid containing 

 sugar, any liquid which is formed by pressing out the 

 succulent parts of the fruits of plants, or a mixture of honey 

 and water, if left to itself for a short time, begins to undergo 

 a peculiar change. No matter how clear it might be at 

 starting, yet after a few hours, or at most a few days, if 

 the temperature is high, this liquid begins to be turbid, 

 and by-and-by bubbles make their appearance in it, and a 

 sort of dirty-looking yellowish foam or scum collects at the 

 surface ; while at the same time, by degrees, a similar kind 

 of matter, which we call the " lees," sinks to the bottom. 



The quantity of this dirty-looking stuff, that we call the 

 scum and the lees, goes on increasing until it reaches a certain 

 amount, and then it stops ; and by the time it stops, you 

 find the liquid in which this matter has been formed has 

 become altered in its quality. To begin with it was a mere 

 sweetish substance, having the flavour of whatever might 



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