224 METHODS IN TEACHING 



it is formed inside the paper tube? What prevents the air 

 from entering the tube? Why does not the flame travel 

 through a gas pipe when we light the gas at the burner? 

 Collect some ordinary illuminating gas by displacement in 

 a wide-mouthed jar; a quart fruit jar will answer. Light 

 the pure gas and note that it burns rather slowly from the 

 top downward. Repeat, by filling the jar with about one 

 fourth gas and three fourths air. Light with a long taper 

 and note that the whole mixture flashes at once. In this 

 connection discuss the blow-pipe, also the bunsen gas 

 burner, and the common gas stove burners, which have 

 devices for mixing the gas with air and thus developing 

 great heat by securing more rapid combustion. 



Verify the taper experiments by placing a small quantity 

 of torn paper in a baking-powder can (not soldered), and 

 heating it over a gas or alcohol flame. When the gray 

 vapor begins to rise in considerable quantity, test with 

 lighter. Does it flash as before? What produces the 

 flame, the solid paper or the gas that is driven off by the 

 heat? Repeat the experiment, substituting small pieces of 

 wood for the paper. Compare results with those of the 

 previous experiment. What is left in the can after the 

 vapor ceases to be given off? Compare with common char- 

 coal. Can this be burned ? Will it produce a flame ? 

 Heat until incandescent, and catch some of the gas that is 

 now given off by inverting a glass over it for a moment. 

 Test the contents of the glass for carbon dioxide. The 

 charcoal can be burned in shorter time if air is admitted 

 by perforating the can near the bottom for this last experi- 

 ment. 



The experiment can be made a little more striking by 



