THE VITAL PHENOMENA OF BACTERIA. 125 



try, although in experiments with animals 

 they can be unmistakably detected. For these 

 reasons we can and indeed must make our 

 conception of these poisons, the " active pro- 

 teids," a wholly provisional one. They are 

 certainly to be referred to the coagulable pro- 

 teids, and are less resistant to heat than the 

 poisons belonging to the group of albumoses 

 and peptones. Their extraordinary " activity " 

 recalls that of the enzymes, and for this rea- 

 son Roux has considered that the diphtheria 

 poison, and Nencki that all these poisons are 

 of an enzyme character. But to regard them 

 as enzymes does not altogether free us from 

 difficulty. The fact of their extraordinary 

 activity still faces us, whether we call these 

 poisons enzymes in the earlier limited sense in 

 which that term was used, or whether, on ac- 

 count of our more extended knowledge of 

 activity on the part of proteid bodies, we fall 

 in with Nencki's suggestion and widen our 

 conception of the nature of an enzyme. 



Effect of Bacteria upon the Substratum. 



From the earliest observers (among whom I 

 need mention only Leeuwenhoek, O. F. Muller, 

 von Gleichen-,Russworm, Ehrenberg and Du- 

 jardin) of " infusion animals," the name under 

 which for a long time bacteria were classed, 



