YEAST 425 



is the result of nothing but the vital activity of this little 

 fungus, the torula. 



And now comes the further exceedingly difficult inquiry 

 how is it that this plant, the torula, produces this singular 

 operation of the splitting up of the sugar ? Fabroni, to 

 whom I referred some time ago, imagined that the efferves- 

 cence of fermentation was produced in just the same way 

 as the effervescence of a sedlitz powder, that the yeast was 

 a kind of acid, and that the sugar was a combination of car- 

 bonic acid and some base to form the alcohol, and that the 

 yeast combined with this substance, and set free the carbonic 

 acid ; just as when you add carbonate of soda to acid you 

 turn out the carbonic acid. But of course the discovery of 

 Lavoisier that the carbonic acid and the alcohol taken 

 together are very nearly equal in weight to the sugar, com- 

 pletely upset this hypothesis. Another view was therefore 

 taken by the French chemist, Th6nard, and it is still held by 

 a very eminent chemist, M. Pasteur, and their view is this, 

 that the yeast, so to speak, eats a little of the sugar, turns 

 a little of it to its own purposes, and by so doing gives such 

 a shape to the sugar that the rest of it breaks up into carbonic 

 acid and alcohol. 



Well, then, there is a third hypothesis, which is maintained 

 by another very distinguished chemist, Liebig, which denies 

 either of the other two, and which declares that the particles 

 of the sugar are, as it were, shaken asunder by the forces 

 at work in the yeast plant. Now I am not going to take you 

 into these refinements of chemical theory, I cannot for a 

 moment pretend to do so, but I may put the case before you 

 by an analogy. Suppose you compare the sugar to a card 

 house, and suppose you compare the yeast to a child coming 

 near the card house, then Fabroni' s hypothesis was that the 

 child took half the cards away ; Thenard's and Pasteur's 

 hypothesis is that the child pulls out the bottom card and 

 thus makes it tumble to pieces ; and Liebig's hypothesis 

 is that the child comes by and shakes the table and tumbles 

 the house down. I appeal to my friend here (Professor 

 Roscoe) whether that is not a fair statement of the case. 



Having thus, as far as I can, discussed the general state 

 of the question, it remains only that I should speak of some 

 of those collateral results which have come in a very remark- 

 able way out of the investigation of yeast. I told you that 

 it was very early observed that the yeast plant consisted of 

 a bag made up of the same material as that which composes 



