To show that carbon dioxide is necessary for photosynthesis.
Take two young potted plants which have been kept in the dark to make the leaves starch-free. Place each of them under a large bell-jar, and place these on vaselined glass sheets. Along with one of the plants is placed a lighted candle which will use up the oxygen under the jar by combustion and replace it .with carbon dioxide. Beside the other plant is placed a dish of caustic soda to absorb the carbon dioxide in the air under the bell-jar.
After leaving these two in the light for a few hours, some leaves are removed from each plant and tested for stajch. It is observed that the leaves of the plant surrounded by plenty of carbon dioxide show the presence of starch, whereas the others without carbon dioxide have no starch.
Therefore carbon dioxide is necessary for photosynthesis.