82 PHYSIOLOGY OF BACTERIA 



the species, the composition of the medium decides 

 whether an organism can grow without oxygen, and 

 whether it can tolerate the toxicity of oxygen. 



IV. THE OXIDATION— REDUCTION POTENTIAL 



(a) ELECTROMETRIC MEASUREMENTS OF POTENTIALS 



In 1911, Potter conceived the idea that, since bio- 

 chemical manifestations are frequently accompanied by 

 electrical phenomena, bacterial cultures might also show 

 a change of potential. He tested this assumption by 

 filling a porous cylinder with a liquid medium and placing 

 it into a glass jar filled with the same medium. One 

 of the two portions of medium was inoculated with a 

 microorganism. Platinum electrodes were then placed 

 into both liquids and an electric current of a certain 

 voltage was actually observed, the whole resembling 

 a galvanic cell. 



The differences in potential were found to be with 



Volts 



Bad. coll in tartrate-asparagin solution . 308 



Bad. coli in tartrate-starch solution at 30°C 0.349 



Bad. coli in tartrate-starch solution at 20°C 0.534 



Yeast in glucose or sucrose solution . 300-0 . 400 



Potter concluded from this evidence that the disintegra- 

 tion of organic compounds by microorganisms is accom- 

 panied by the liberation of electrical energy. 



Nine years later, Gillespie (1920) showed that the 

 electric potentials observed by Potter were only a special 

 case of the oxidation-reduction potential. If two plati- 

 num electrodes are placed in the two open ends of a U- 

 tube filled with acid, and one electrode is saturated with 

 hydrogen and the other with oxygen, the difference 

 in potential of the two electrodes would be 1.23 volts. 



