ENERGY SUPPLY OF THE CELL 85 



at the pH of the culture under test. If the hydrogen 

 pressure is less than 1 atmosphere, the reduction intensity 

 will be lower, and it can be computed how much lower 

 it will be for any given hydrogen pressure. This leads 

 to the conception of Th (see p. 90). 



Gillespie's data show that in one case, Bad. coli produced the 

 same potential as the hydrogen electrode; in other words, this cul- 

 ture reduced as strongly as hydrogen gas at 1 atm. pressure, and, 

 consequently, could develop hydrogen gas. The differences between 

 hydrogen potential and reduction potential of the other two cultures 

 of Bad. coli correspond to hydrogen pressures of 0.34 and 0.052 atm. 

 respectively. 



The potentials mentioned above are differences in 

 potential between the culture and an arbitrary standard, 

 in one case the sterile medium, in the other case a calomel 

 half cell. There is no absolute zero point for electric 

 potentials, but in order to compare potentials, a zero 

 point is fixed arbitrarily. The potential of the normal 

 hydrogen electrode is generally accepted as the standard 

 zero point. This may be compared to fixing the zero 

 point of the thermometer scale by the freezing point of 

 water. The difference in potential between the normal 

 hydrogen electrode and the saturated calomel electrode 

 is 0.247 volts, and this amount must be added to measure- 

 ments with the saturated calomel electrode to make them 

 comparable with the standard scale. The EMF on 

 the standard scale is usually expressed as Eh. 



The Eh of most sterile media in air is about +0.2 

 to 0.3 volts, and becomes negative by the action of bac- 

 teria. Therefore, the curves' of potential change in 

 bacterial cultures will show a decrease of potential if 

 plotted on the standard or Eh scale. The lower the 

 potential, the lower is the oxidizing intensity, and the 

 stronger is the reducing intensity. If we plot potential 



