194 LECTURE XI. 



proteids which uridergo putrefaction in the presence of Bac- 

 teria, are assimilated by these organisms, that they are built- 

 up into living protoplasm, and that the characteristic products 

 of the decomposition of these substances are set free in conse- 

 quence of the self-decomposition of the protoplasm-molecules 

 of the organisms, for the weight of the organisms formed is but 

 a small fraction of the weight of the substances decomposed 

 in these processes. Taking the alcoholic fermentation for 

 example, Pasteur has shewn that for a weight a of Yeast-cells 

 formed, a weight of 100 a or more of sugar may be decomposed, 

 and, if similar determinations were made with respect to the 

 other fermentations, the result would probably be the same. 

 Nor is it more easy to believe that these organisms contain 

 substances of the nature of unorganised ferments which effect 

 the chemical changes peculiar to them ; that Yeast, for in- 

 stance, contains an unorganised ferment which is capable 

 of decomposing sugar into carbon dioxide and alcohol, or 

 that Bacteria contain some sort of trypsin by means of which 

 they decompose proteids. As a matter of fact all attempts 

 to extract such substances from Yeast and Bacteria have met 

 with no success, and, when we bear in mind that these fermen- 

 tations only go on in the presence of living organisms, the 

 existence of such substances is rendered extremely impro- 

 bable. It is urged, however, by those who hold that the 

 fermentations are effected by unorganised ferments produced 

 by the organised ferments, that these unorganised ferments 

 may be extremely unstable and that their formation may 

 only go on so long as the organised ferment is living. But 

 this suggestion by no means accords with our present know- 

 ledge of unorganised ferments, for they have been found to 

 be remarkably stable ; we must therefore regard this interpre- 

 tation of the phenomena of fermentation as highly hypo- 

 thetical. 



But if we reject these two explanations of the nature of 

 fermentation, what other more satisfactory explanation can 

 be offered ? It seems to be a fair and unstrained inference 

 from the facts before us, that living protoplasm, besides 

 undergoing decomposition itself, can induce decomposition 



