132 EVOLUTION OF OXYGEN [CH. 



with the practically unlimited supplies of carbon-dioxide 

 available in the atmospheric ocean passing over and in 

 contact with them, and with the experimental proof that 

 no other source of carbon is open to the green plant, to 

 prove that the latter nevertheless does accumulate carbon 

 to the extent stated. 



Long before this was known, however, observations 

 had established that green leaves in sunlight evolve pure 

 oxygen gas. The fundamental experiment is easily re- 

 peated. A green water-weed, e.g. Elodea, is placed in water 

 in a thin glass beaker, and a funnel placed mouth down wards 

 covers the plant. Over the leg of the funnel an inverted 

 test-tube full of water is placed, and the whole exposed to 

 bright sunshine. In a few minutes bubbles of gas ascend 

 the funnel and pass into the tube, displacing water, and 

 accumulate in the closed upper end of the test-tube. In 

 a few hours the tube may be filled with the gas, and the 

 ordinary tests ignition of a glowing match, absorption by 

 pyrogallic acid prove it to be oxygen. 



The decomposition of carbon-dioxide in the chlorophyll- 

 apparatus by the agency of sunlight is, then, as might be 

 inferred, attended by the liberation of free oxygen; and as 

 it can be shown that, in the normal process, the volume 

 of oxygen liberated is equal to that of carbon-dioxide 

 decomposed, it is possible to utilise the foregoing experi- 

 ment for estimating the rate of the process, as follows. 



If the stalk of an Elodea plant, well provided with 

 leaves, is cut clean across, the escaping oxygen bubbles 

 ascend at regular intervals into the funnel, and can be 

 counted so many bubbles per minute. On shading the 

 apparatus the number diminishes ; on intensifying the 

 illumination it increases, and so on, within the limits 

 of temperature and other conditions. The method of 

 " bubble-counting " can therefore be employed to ascertain 



