2O4 METHODS IN TEACHING 



attention of the pupils should be called to this fact. Im- 

 merse the free end of the glass tube in a shallow dish of 

 water and clasp the bulb (flask) with the hands. Almost 

 immediately bubbles of air begin to escape from the tube. 

 Why? Hold the bulb some distance above a Bunsen 

 burner or an alcohol lamp ; heat carefully, until as much of 

 the air as possible has been driven out. Keeping the end 

 of the tube submerged in water, allow the bulb to cool 

 slowly; note that water travels up through the tube into 

 the flask. Why? Now empty the water from the flask 

 and tube. Warm the bulb as before by clasping the hands 

 about it. When a small amount of air has been forced out, 

 let the bulb cool enough to suck up three or fouf inches 

 of ink or colored water. Hold the flask upright and let the 

 water sink through the tube almost to the bulb. The in- 

 strument will now be found to be very sensitive to the 

 slightest change of temperature in the bulb, the short col- 

 umn of water being forced up or down through the tube 

 by the expansion or contraction of the air in the flask. 

 With a little practice in handling this " air thermometer," 

 very interesting, positive results may be obtained. In such 

 experiments pupils should furnish the explanations for the 

 phenomena observed. They should also be able to explain 

 the popping of corn and chestnuts when heat is applied, 

 the blowing out of bicycle tires when left in the hot sun, 

 and similar occurrences. 



For showing the expansion of liquids the same apparatus 

 can be used. Fill the flask full of water that has been 

 boiled to remove the air and afterward allowed to become 

 cold. Press the stopper into the mouth of the flask until 





