290 Biological Chemistry. 



to the presence of living organisms, many of which can 

 be rendered visible only by the highest powers of the 

 microscope. Where matter capable of undergoing fer- 

 mentative or putrefactive changes exists under such con- 

 ditions that those organisms have been first destroyed 

 and have no subsequent access, it will remain " sterile," 

 and be " preserved " against putrefaction for an indefinite 

 period. 



The next questions which arose were Was it possible 

 to produce fermentative or putrefactive changes without 

 the presence of the living agent ? Was it possible to kill 

 the living yeast cell, for example, and then to extract 

 some substance from it which could cause the conversion 

 of sugar into alcohol and carbon dioxide ? Reasons 

 existed for supposing that the above questions could be 

 answered in the affirmative. It had been shown earlier, 

 by Dubrunfaut, that germinating barley possessed the 

 power of converting starch into sugar, and in 1833 Payen 

 and Persoz found that the aqueous extract of the plants 

 yields on the addition of alcohol a white powder which 

 also possessed a capacity for vigorously degrading starch. 

 Many other analogous products were discovered subse- 

 quently, such as the starch-digesting substance from the 

 saliva, and the protein-digesting substance from the gastric 

 juice. In all cases the reaction was brought about by the 

 addition of relatively very small amounts of the active 

 substance. In the case of yeast, it suffices to inoculate 

 the sugar solution with a few yeast cells to produce 

 action ; sterile vegetable or animal matter will putrefy 

 after coming into contact with air which has not been 

 purified, and which contains only a very minute amount of 

 foreign substance ; the addition of a very small amount 

 of the active powder obtained from germinating barley 

 will convert a relatively very large amount of starch into 



