FUNCTIONS OF LEAVES 167 
Carbon dioxide is produced in immense quantities by 
the decay of vegetable and animal matter, by the respira- 
tion of animals, and by all fires in which wood, coal, gas, 
or petroleum is burned. 
Green leaves and the green parts of plants, when they 
contain a suitable amount of potassium salts, have the 
power of removing carbon dioxide from the air (or in 
the case of some aquatic plants from water in which it is 
dissolved), retaining its carbon and setting free part or all 
of the oxygen. This process is an important part of the 
work done by the plant in making over raw materials into 
food from which it forms its own substance. 
EXPERIMENT XXXIV 
Oxygen-Making in Sunlight. — Place a green aquatic plant in a 
glass jar full of ice-cold fresh water, in front of a sunny window.! 
Place a thermometer in the jar, watch the rise of temperature, and 
note at what point you first observe the formation of oxygen bub- 
bles. Remove to a dark closet,for a few minutes and examine by 
lamplight, to see whether the rise of bubbles still continues. 
This gas may be shown to be oxygen by collecting some 
of it in a small inverted test-tube filled with water and 
thrusting the glowing coal of a match just blown out into 
the gas. It is not, however, very easy to do this satisfac- 
torily before the class. 
Repeat the experiment, using water which has been well boiled 
and then quickly cooled. Boiling removes all the dissolved gases 
from water, and they are not re-dissolved in any considerable quantity 
for many hours. 
1 Elodea, Myriophyllum, Chrysosplenium, Potamogeton, Fontinalis, any of 
the green aquatic flowering plants, or even the common confervaceous plants, 
known as pond-scum or “‘ frog-spit,’’ will do for this experiment, 
