126 



THE LIFE OF THE PLANT 



stopper, taking care not to leave air bubbles below it. 

 Then we place the apparatus in the light. As before, 

 the leaf becomes covered with minute 

 bubbles, and these, after reaching a 

 certain size, rise to the upper part of 

 the tube ; here there accumulates a per- 

 ceptible and ever-increasing quantity 

 of gas. While the volume of gas 

 constantly increases in the right-hand 

 side of the tube the volume of car- 

 bonic acid diminishes in the left. At 

 the moment when the level of the 

 water in the right-hand side reaches 

 a' the level in the left-hand side will 

 also be at a. Evidently the gas in the 

 right-hand side is oxygen ; but to be 

 quite sure, we can withdraw the 

 stopper and test the gas with a 

 splinter. Once having proved that 

 it is indeed oxygen, we refill the tube 

 with water and repeat the experiment. 

 Again we get a certain quantity 

 of oxygen while a corresponding 

 quantity of carbonic acid disappears 

 from the other side of the tube. We 

 know it is carbonic acid because we introduced it our- 

 selves, but for greater certainty, after several similar 

 experiments, we fill up the right-hand side of the tube 

 with water, replace the stopper and, turning the whole 

 tube upside down, transfer the remaining gas from the 

 left into the right-hand side. If we test this gas we 

 find not only that a smouldering splinter does not glow 

 in it but that even a burning one will be extinguished. 

 This means that the gas was and remains carbonic acid. 

 What happens in this experiment is easily understood : 

 carbonic acid continually dissolves in water in the left- 

 hand side of the tube ; but the solution in the right arm 



Fig. 36. 



