6o 



BURNING OR OXIDATION 



to keep the atmosphere free from an ex- 

 cess of carbon dioxide and in addition, 

 they furnish oxygen to the atmosphere. 

 This can be shown by a simple experi- 

 ment. Place a glass funnel over a plant 

 growing in a sunny aquarium. Then fill 

 a test tube with water and invert it over 

 the funnel as shown in Figure 30. Bub- 

 bles of gas slowly make their way up the 

 funnel and force some of the water 

 out of the test tube. If the test tube is 

 removed and a smoldering splinter is 

 thrust into the gas, the splinter bursts 

 into flame, showing that oxygen is 

 present. The oxygen has come from 

 the plant. Experiments also prove that 

 plants absorb carbon dioxide. Plants 

 serve a twofold good ; they remove the 

 injurious gas, carbon dioxide, and they 

 set free the desired gas, oxygen. 

 How to obtain carbon dioxide. There are several ways in 

 which carbon dioxide can be produced commercially, but for 

 laboratory use, the simplest 

 way is to mix powdered 

 marble, or chalk, and hydro- 

 chloric acid in a test tube, 

 and to collect the effervesc- 



inggas asshowninFigure3i. ^ ,. W A 



The escaping carbon dioxide 

 is heavier than air, and hence 

 it settles in A. The sub- 

 stance which is left in the 



FIG. 31. Making carbon dioxide from 

 test tube after the gas has marble and hydrochloric acid. 



FIG. 30. Plants give 

 off oxygen. 



