XXIV 



GENERAL INTRODUCTION 



Place a bit of quicklime in about half a glass of water 

 on the day previous to the experiment. When ready for 

 use there will be a white sediment at the bottom and a thin 

 white scum on the top of the clear lime- 

 water. The pupils should see this white 

 scum, as a question about it will follow. 

 Make a loop in the end of the piece of 

 wire by turning it around the point of a 

 lead pencil. Remove the scum from the 

 limewater with a piece of paper and insert 

 the loop into the clear water. When 

 withdrawn, the loop ought to hold a film 

 of clear water. Pass the wire through a 

 FENDED FILM piece of cardboard or stiff paper, and 

 OF LIMEWATER. arrange as s hown in D. 



Place the chimney over the lighted candle. Lower the 

 loop into the chimney and cover the top of the chimney 

 with the paper. Withdraw the wire two minutes after the 

 candle goes out. Note the cloudy appearance of the film 

 of water on the wire. The cloudiness was caused by the 

 carbon dioxid formed while the candle was burning. 



Omitting the candle, hang the freshly wetted wire in the 

 empty chimney. Let the film of limewater remain within 

 the chimney for the same length of time as when the can- 

 dle was used. It does not become cloudy now. The 

 cloudiness in clear limewater is a test or indication that 

 carbon dioxid is present. 



What caused the white scum on the limewater which 

 stood overnight ? 



How does the CO 2 get into the air ? It is formed when- 

 ever wood, coal, oil, or gas is burned. 



The amount of CO 2 in ordinary air is very small, being 

 only three parts in ten thousand. If the limewater in the 



