1 84 THE SCIENCE OF LOGIC 



any length of time [ If not A (air) then not F ], but that if a single bubble 

 of air were admitted fermentation soon appeared [ If A then F ].&quot; This 

 careful experimental application of the method of single difference merely 

 proved that something in the air actually admitted, and therefore in all 

 similar air admitted in similar circumstances, causes fermentation. The sup 

 position that this &quot; something &quot; consists in living germs had yet to be proved 

 by negative experiments that would presumably rid the air of such germs. 

 (2) &quot; Schwann repeated Gay-Lussac s experiment and showed that if the air 

 were admitted to the vacuum through a red-hot tube then fermentation did not 

 occur.&quot; This proved that the &quot; something &quot; could be destroyed by great heat, 

 and so went to strengthen the probability of the proposition &quot; If not G, then 

 not F&quot;. (3) Still further probability was added by various experiments which 

 went to show that a &quot; temperature of from 20 C. to 24 C. was most favour 

 able to &quot; fermentation ; &quot; while the process was stopped at freezing point 

 (o C.) and again at 60 C. ; and boiling destroyed it&quot;. (4) &quot;Afterwards 

 Helmholtz showed that oxygen produced by electrolysis in a sealed-up tube 

 containing a boiled fermentable fluid did not cause fermentation.&quot; Here 

 again, the probability of the proposition &quot; If not G, then not F&quot; is increased, 

 inasmuch as the pure oxygen so obtained differs from ordinary atmospheric air 

 in the absence of all extraneous organic impurities, living germs included. 

 (5) &quot; Hoffmann showed that air filtered through cotton wool was incapable of 

 causing fermentation &quot; the air being presumably purified of all organic mat 

 ter, including living germs, by such filtering. Therefore, again, the proposi 

 tion &quot; If not G, then not F&quot; was corroborated. (6) Better than all the fore 

 going negative instances was the one secured by Helmholtz in the following 

 application of single difference, with its simultaneous positive and negative 

 instances : &quot; He placed a sealed bladder full of grape juice in a vat of fer 

 menting juice, and found that the fluid in the bladder did not ferment. Thus 

 the cause of the fermentation could not pass through the bladder. If the 

 fermentation were excited, as Liebig held, by a separate substance formed by 

 the yeast cells, and presumably soluble, one would have expected it to pass 

 through the wall of the bladder ; but if the process were caused by the small 

 yeast cells, then one can see why fermentation was not excited, as the yeast 

 cells could not pass through the membrane.&quot; This experiment tended to 

 disprove the hypothesis of a soluble substance as the immediate cause of fer 

 mentation. It did not, however, prove Pasteur s view, that fermentation is not 

 a merely chemical process taking place outside the yeast-cells, but rather a 

 vital or physiological process which takes place within them and transforms the 

 assimilated sugar into alcohol and other products. So far as was yet known, 

 fermentation might be due to the chemical action of some insoluble product of 

 the vital functions of the yeast cell. Subsequent experiments did bring to 

 light a substance of this sort, namely zymase, to the action of which fermenta 

 tion seems to be immediately due : a substance which, however, can be 

 produced only by the vital functions of living cells. 1 



1 &quot; In 1897 Buchner submitted yeast to great pressure, and isolated a nitrogenous 

 substance, enzymic in character, which he termed zymase . This body is being 

 continually formed in the yeast cell, and decomposes the sugar which has diffused 

 into the cell. ... In this respect the plasma behaves in a similar manner towards 



