THE INDUCTION-PERIOD 87 



period described above may be due not to chemico-physio- 

 logical action but to certain other causes ; that during the 

 prolonged period of darkness the oxygen which filled the 

 intercellular spaces might have been removed either by 

 absorption by the water or in respiration, and it would then 

 take some time for the empty spaces to be filled up once more 

 with oxygen : that the result is in fact attributable, wholly 

 or in part, to an absorption-respiration factor. This 

 plausible explanation is, however, unsatisfactory, since both 

 the processes of absorption of oxygen by water and loss by 

 respiration are relatively slow. There are, moreover, two 

 independent tests by which the conclusion that the pro- 

 longation of the induction-period is in fact a chemico- 

 physiological phenomenon is established. I describe here 

 the first of these tests ; the second, namely the effect of 

 successive intermittent illuminations of short duration, 

 will be treated in the next chapter. If the prolongation 

 of the period for the appearance of the first bubble on re- 

 exposure after previous darkness were simply due to the 

 delay in filling the intercellular spaces with oxygen, then, 

 as these spaces are already filled at the evolution of the first 

 bubble, the subsequent second and third bubbles should 

 be at normal intervals. If, on the other hand, the delay 

 were due to physiological inertia in the sense already 

 described, the delay would persist for a short time : the 

 evolution of the second, and probably of the third, bubble 

 would exhibit a gradually diminishing retardation till the 

 normal rate was restored. 



The following experiment demonstrates the existence 

 of the physiological factor. The rate of appearance of 

 successive bubbles under continuous light from the sky 

 was 20 seconds. The light was cut off for periods which 

 increased from i to 5 and 10 minutes (see table on p. 88). 



It will be noted that, while the induction-period was in 

 the three cases increased from 20 seconds to 48, 85 and 120 

 seconds, the second bubbles also exhibited a retardation, 

 the periods being 21, 30 and 70 seconds respectively. It 



