w. D. Mcelroy and j. w. Hastings 



185 



ture returns after the flash again corresponds to the intensity level 

 which the control reaction had arrived at in the same time (Fig. 17). 

 If the reactions that lead to the formation of the inactive intermediate 

 did not occur under oxygen-free conditions, one would expect that 

 the reaction would proceed as the control from the time air is first 

 admitted. 



2 3 



TIME - MINUTES 



Fig. 18. Effect of various oxygen concentrations on the time course of the light 

 reaction. Air was admitted to obtain the flash height values indicated. 



By following the time course of the light reaction at various oxygen 

 tensions it is possible to show that the initial flash intensity is de- 

 pressed by oxygen tensions which do not influence the steady-state 

 light level (Fig. 18). The results indicate that a rapid utilization of an 

 intermediate by oxygen is limiting in the initial reaction, whereas the 

 formation of this intermediate is limiting after the steady state is 

 reached. In the experiments recorded in Fig. 18, oxygen was admitted 



