THE LEAF 131 



property of absorbing oxygen, becoming dark brown in 

 colour during the process. We lift up the rod, and as 

 soon as the first drops of this liquid penetrate into the 

 tube and come into contact with the gas contained in it, 

 the liquid becomes coloured and the volume of the gas 

 rapidly decreases. In the end we shall have something 

 like fifteen divisions of gas instead of forty-eight. This 

 remaining gas is nitrogen, and this means that we had 

 thirty-three parts of oxygen. Not more than five parts 

 of oxygen will have penetrated with the fifteen parts of 

 nitrogen in the form of atmospheric air, so that twenty- 

 eight out of thirty-three divisions represent oxygen 

 given off by the plant owing to the decomposition of 

 carbonic acid. 



The apparatus we have just described, together with 

 electric light, makes it possible for us to-day to demon- 

 strate before an audience on a dark winter evening a 

 phenomenon that generally takes place in nature only 

 in the daytime during the warm season of the year ; we 

 can throw it upon the screen as easily as an ordinary 

 lantern slide. Of course the apparatus can also be 

 used without the lantern as a simple and convenient 

 method of investigation. 



So far we have been studying the decomposition of 

 carbonic acid by plants submerged in water. This 

 form of experiment is the most convenient for a pre- 

 liminary study of this phenomenon, because it clearly 

 shows the giving off of gases by the plant. We must 

 now ascertain whether the same kind of decomposition 

 also takes place as the result of contact of the leaf 

 with air which contains carbonic acid. 



Here is a very rough and simple form of experiment, 

 by which the phenomenon was demonstrated for the first 

 time a hundred years ago by the great chemist Priestley. 

 We take a glass jar (as in fig. 34), pour into it a small 

 quantity of water and fix at the bottom of the jar a 

 lighted bit of candle, of course large enough to stick up 



