GRADUAL DEVELOPMENT OF THE GERM THEORY. 71 



different fermentations are caused by different kinds of 

 micro -organisms. 



The strict proof that living yeast cells or minute Evidence re- 

 organisms similar to yeast are the only cause of every prove^he cb- 

 fermentation, could however only be furnished by a pendence of 



y r fermentation 



series of experimental investigations, which must have on living 

 for their aim the following questions : 



1. In the first place it was necessary to show that i. Living cells 

 germs are present in all fermenting and putrefying fnan'fennent- 

 fluids. This was proved by all the investigators who in fluids - 

 busied themselves with the question of fermentation 



after Schwann, and in fact it was the constant presence 

 of definite microscopical organisms which formed the 

 starting point of the vitalistic theory. The fact itself 

 was much less disputed by the adversaries of this theory, 

 than its meaning. Not till later years were here and 

 there observations published which asserted the exist- 

 ence of putrefying and fermenting media without micro- 

 organisms observations which will be considered in 

 detail later. 



2. From the constant concurrence of putrefaction and 2. Fermen- 

 micro-organisms the causal role of the latter did not stances do not 



of course directly follow; that must on the 



be proved by special experiments. The behaviour of of living yeast 



ferinentescible substances without micro-organisms was vented. 



therefore tested in the first instance ; and for this 



purpose the attempt was made to kill any germs which 



might be present in the substances themselves, in the 



vessels, &c., by a temperature of at least 100 C., care 



being then taken to protect the materials against the 



entrance of fresh germs, either by suitable methods of 



closing the vessels, or by subjecting the entering air to 



the action of means which could kill the germs. 



These experiments also date back to an early period. 

 In 1836 F. Schulze showed that no decomposition 

 occurred in putrescible materials when he boiled them, 

 thus killing any germs which might be present, and then 

 prevented the access of air, as for example by inter- 

 posing a layer of oil, or conducted the entering air 

 through sulphuric acid, which detained and destroyed 



