78 AIR 



a. Fill bottle with pure water, and empty it in the 

 place where the air is to be tested. This will insure the 

 bottle being filled entirely with the air of the room. Add 

 10 c. c. of limewater solution and one-third c. c. of 

 phenolphthalein solution, stopper, and shake. If the red 

 color disappears in three minutes or less, the air is unfit 

 to breathe. 



30. Oxygen and its Uses. 



We have learned that air is necessary for life of any 

 kind and that it is composed of nitrogen, oxygen, carbon 

 dioxide and water. We have also learned that it is the 

 oxygen which caused the candle to burn since it stopped 

 burning when all the oxygen had been used. Therefore 

 we call oxygen the supporter of combustion. Just as the 

 candle needs oxygen in order to burn, so we need oxygen 

 in order to live. It it only the oxygen of the air which 

 the body uses ; the nitrogen is exhaled unchanged. If 

 there are many persons in a close room most of the oxy- 

 gen will soon be used and the air will become unfit for 

 breathing. Headaches and drowziness are more often 

 produced by bad air than by hard lessons. Yawning is 

 caused, by a desire to obtain more oxygen. 



Since it is the oxygen of the air which supports com- 

 bustion and air is composed of only one-fifth oxygen, we 

 might suspect that combustibles would burn much more 

 vigorously in oxygen than air. Let us see if this is true. 



Experiment 40. To Prepare and Use Oxygen. 

 Apparatus: Test tube, test-tube holder, burner. 



