2l8 METHODS IN TEACHING 



I 



flame extinguished? If no one is able to answer, explain, 

 and repeat the experiment. Why does the water rise as the 

 candle burns and for a short time after it is extinguished? 

 Has any part of the air been actually used ? Can anything 

 be burned in it now ? Slip a piece of pasteboard under the 

 mouth of the jar and hold it in place while the jar is turned 

 right side up. Light a taper and thrust it into the jar. 

 Does it burn? No. Now invert the jar over a piece of 

 burning candle on the table. Can air get into it now? 

 The candle burns for a time and then goes out. Lift the 

 jar gently while still warm, cover, and turn right side up. 

 Pour into the jar about a tablespoonful of clear lime water, 

 and shake it thoroughly. Note that it turns milky, showing 

 the presence of carbon dioxide. Explain the lime water 

 test to pupils, and as a control experiment, shake some lime 

 water in a vessel filled with pure air. The absence of milki- 

 ness indicates that, if carbon dioxide is present at all in 

 pure air, it is only in very minute quantity. As a matter 

 of fact, it is about five parts in ten thousand. 



Fill a jar with water, place a card over its mouth, and 



invert into a vessel which already contains an inch or two 



of water with a capacity for at least as 



Collecting much more as the jar contains. Insert the 



Defacement end of the deliveI 7 tube under the ed g e 

 of the inverted jar, and the gas as gener- 

 ated will displace the water. Collect Jb^ the above method 

 a jar of breathed air, and test for oxygen and carbon diox- 

 ide: the former, by means of the lighted taper; and the 

 latter, with lime water. Repeat several times, if necessary, 

 until the pupils see that air that has supported burning and 

 air that has been breathed are very much alike in the fact 



