98 PHYSIOLOGY OF BACTERIA 



ments with different specimens of broth, it approached a 

 value between —0.050 and —0.060 volts. This is the 

 potential of cysteine which is likely to be present in 

 meat infusion in very small quantities. 



Fig. 10. — Gradual drop of potential in sterile broth after 5 hours of deaeration 

 with nitrogen gas. 



Dubos (1929) determined by means of indicators the reduction 

 potential of meat infusion broth made with Fairchild's peptone, and 

 he observed decolorization as far as indigo disulfonate (see Fig. 7) 

 which means a potential lower than —0.125 volts. The quantity 

 of dye that could be reduced was only 0.0017 molar. Thornton and 

 Hastings (1929) found the potential of fresh milk to be between 

 +0.200 and +0.300 volts. The final potential of deaerated milk is 

 sufficiently low to decolorize methylene blue. The addition of 

 cysteine brought the potential of raw milk from 0.2 to volts in 

 fifteen minutes, and to —0.2 volts in three hours. At this time, 

 the check sample without cysteine had just started on the decrease, 

 and had gone from +0.2 to +0.15 volts. The final potential of 

 both samples was —0.2 volts, but this may have been due to bacteria 

 and not to the milk as such. 



Since each medium has a characteristic reduction 

 potential which establishes itself some time after the 

 removal of dissolved oxygen, and since all aerobic and 



