416 BOTANICAL BIOLOGY. 



contaiued iu the fruits of an ludiau plant, Witha7iia coagulcms, and ad- 

 mirably investigated by Mr. Sheridan Lea. 



Having dwelt so far on the action of ferments, we may now turn to 

 fermentation, and that other kind of change in organic matter called 

 " putrefaction, " which is known to be closely allied to fermentation. 

 Ferments and fermentation, as I have already remarked have very little 

 to do with one another; and it would saveconfusion and emphasize the 

 the fact if we ceased to speak of ferments but used some of the altern- 

 ative names which have been proposed for them, such as zymases or 

 e7izy7nes. 



The classical case of fermentation, which is the root of our whole 

 knowledge of the subject, is that of the conversion of sugar into alcohol. 

 Its discovery has everywhere accompanied the first stages of civilization 

 iu the human race. Its details are now taught iu our text books ; and 

 I should hardly hope to be excused for referring to it in any detail if it 

 were not necessary for my juirpose to draw your attention more partic- 

 ularly to one or two points connected with it. 



Let us trace what happens in a fermenting liquid. It becomes turbid, 

 it froths and effervesces, the temperature sensibly increases ; this is the 

 first stage. After this it begins to clear, the turbidity subsides as a 

 sediment ; the sugar which the fluid at first contained, has in great 

 part disappeared, and a new ingredient, alcohol, is found in its place. 



It is just fifty years ago that the great Dutch biologist Schwann made 

 a series of investigations which incontrovertibly demonstrated tbat both 

 fermentation and putrefaction were due to the presence of minute or- 

 ganisms which live and propagate at the expense of the liquids in 

 which they produce as a result these extraordinary changes. The 

 labors of Pasteur have confirmed Schwann's results, and — what could 

 not have been foreseen— have extended the possibilities of this field of 

 investigation to those disturbances in the vital phenomena of liviug 

 organisms themselves which we include under the name of " disease," 

 and which, no one will dispute, are matters of the deepest concern to 

 every one of us. 



Now, at first sight, the conversion of starch into sugar by means of 

 diastase seems strikingly analogous to the conversion of sugar into 

 alcohol. It is for this reason that the phenomena have been so long 

 associated. But it is easy to show that they are strikingly different. 

 Diastase is a chemical substance of obscure composition it is true, but 

 inert and destitute of any vital properties, nor is it affected by the 

 changes it induces. Yeast, on the other hand, which is the active agent 

 in alcoholic fermentation, is a definite organism; it enormously increases 

 during the process, and it appears to me impossible to resist the con- 

 clusion that fermentation is a necessary concomitant of the i^eculiar con- 

 ditions of its life. Let me give you a few facts which go to prove this. 

 In the first place, you can not ferment a perfectly pure solution of sugar. 

 The fermentable fluid muist contain saline and nitrogenous matters 



