FOOD OBTAINED FROM THE AIR DL. 
smaller end of the funnel is below the surface of the water. 
Having filled a narrow test-tube with water, invert it over 
the neck of the funnel; when in position the test-tube 
should be completely filled with water. If this arrange- 
ment be exposed to sunlight, bubbles of gas will be given 
off by the plant and will collect in the tube. When a 
sufficient quantity of this gas has been collected, the test- 
tube can be removed and the gas tested. Be careful to 
close the test-tube- with the thumb before raising it from 
the water. A glowing splinter plunged into the test-tube 
will at once burst into flame showing that the gas is 
oxygen. > 
If now the experiment be.repeated in the dark, or with- 
out using any plant, no gas will be given off. From this 
we learn 
(a) That the green leaves of plants are able to decompose 
carbon-dioxide ; 
(b) That sunlight is necessary for the process. 
The immense quantity of carbon-dioxide continually 
being poured into the air as the product of respiration and 
combustion, as also from the decomposition of organic matter, 
and from other sources, is thus to a large extent withdrawn. 
The leaves decompose the carbon-dioxide, returning the 
oxygen to the air, which in this way is rendered suitable for 
the support of life. 
NotTE.—Owing to the small amount-of carbon-dioxide held in 
solution in ordinary water, it is sometimes difficult to collect 
a sufficient quantity of oxygen to test its properties. It will 
be found a good plan before making the experiment to empty 
a bottle of soda-water into the water in the vessel. When this is 
done the whole should be stirred about or shaken up for a few 
minutes before fixing the apparatus—in order to remove any 
of the carbon-dioxide which is likely to be liberated during the 
progress of the experiment. 
