The Action of Light on Chlorophyll. 



399 



exposed to sunlight. They were kept under careful observation to see 

 that the expansion inside the tubes did not drive out any of the air. 

 The chlorophyll in the chlorophyll tube soon began to bleach, and the 

 level of the water began to rise and, in the course of a few hours, it 

 reached a height of rather more than l/5th of the tube, showing that 

 part of the air, probably the oxygen, had been absorbed. The water in 

 the control tube did not rise. The tubes were allowed to remain in the 

 light until no further rise in the chlorophyll tube took place. At this 

 stage the chlorophyll was not completely bleached, but as on prolonged 

 exposure to sunlight no further bleaching took place, it seemed fair to 

 conclude that all the oxygen had been used up. The tubes were now 

 brought to the back of the room into diffused light and allowed to stand 

 for some hours. Careful measurement of the chlorophyll tube showed 

 that the level of water in the tube had risen to a height corresponding 

 exactly to the percentage of oxygen likely to be present in the air 

 enclosed in the tube when the experiment started. To test this, the control 

 tube was placed in a strong solution of potassium hydrate and pyro. 

 This gradually diffused into the water' contained in the tube and then 

 gradually absorbed the oxygen in the tube. In the course of 24 hours, 

 the level of this solution had risen in the tube until it was stationary, 

 and this was found to be almost exactly the same height as the level of 

 the water in the chlorophyll tube. This showed pretty conclusively that 

 in the bleaching of the chlorophyll the whole of the oxygen of the air had 

 been used up. To show that no oxygen was left, however, the chlorophyll 

 tube was itself placed in the potassium hydrate pyro solution ; the tube 

 was gently warmed until the water was nearly driven out, and then, on 

 cooling, the pyro solution entered the tube and rose to exactly the same 

 level as the level of the water previously contained in the tube, and 

 remained at that level, thus showing that no oxygen had been left in the 

 tube. 



Experiments were then made with the other two tubes which had been 

 kept in the dark ; one was placed in a solution of potassium hydrate and 

 pyro, the other was placed in the sunlight for some hours. On leaving 

 them to stand for some hours at the back of the room to equalise the 

 temperature, the level of the liquid in both was the same. Further 

 experiments conducted more carefully with due attention to the corrections 

 necessary for temperature and pressure showed quite conclusively that 

 oxygen is absorbed when chlorophyll is bleached in the light, and that if 

 sufficient chlorophyll is present, the whole of the oxygen in the air in 

 contact with it is used up. It is possible, in fact, to make use of 



