268 NATURE STUDY AND AGRICULTURE 



such as leaves, grass, corn stalks, brush. Do you re- 

 member seeing any charcoal or carbon ? 



We said the plant gets its carbon out of the air. Do 

 you think it gets it in the form in which you see the carbon 

 in the charcoal? It gets it in the form of a gas called 

 carbon dioxide, CO2. Is this gas an element or a com- 

 pound? How do you know? We may make some 

 carbon dioxide in the following manner. Place a heaping 

 teaspoonful of soda in a glass half full of water and add to 

 this a few teaspoonfuls of vinegar. Stir and see what 

 happens. This gas that bubbles up is carbon dioxide. 

 What does the CO2 tell you about a molecule of the 

 gas? Test for carbon dioxide with limewater. Lime- 

 water may be made by placing a piece of unslaked lime in 

 a bottle, and filling the bottle nearly full of water. Shake 

 well, and set aside for twenty-four hours. The clear water 

 on top is limewater. 



Pour a little of the carbon dioxide from the glass, while 

 the bubbles are coming off most rapidly, into a glass which 

 contains a little limewater. Shake well. What change 

 has taken place? The milky limewater is a sure test for 

 carbon dioxide. Test your breath for carbon dioxide by 

 breathing through a straw or tube into a glass containing a 

 little limewater. 



We have already discussed three elements that plants 

 use in manufacturing food. What are the other things 

 that are needed ? The following is a list of elements that 

 plants need: Oxygen, hydrogen, carbon, nitrogen, sulphur, 

 phosphorus, iron, magnesium, potassium (usually in the 

 form of potash), sodium, chlorine, silicon, calcium. Many 

 plants can get along without sodium, chlorine, and silicon. 



