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express his individual opinion, and to ask his own question. Do not by 

 your conduct with the class put a premium so much on any kind of 

 answers, even bright ones, as on eaj'fiest questions and answers. Give 

 all such, even if apparently ignorant and far from right, every consider- 

 ation. Children, like older people, are sensitive in regard to the respect 

 given to their opinions. Nothing to them is so inimical to the inde- 

 pendent formation of opinion as to have their opinions treated with dis- 

 respect. If one is wrong, prove that he is wrong, but neither ridicule 

 nor ride over him. Avoid allowing a few to lead the class and set opin- 

 ions for the rest. 



HEAT EXPANDS AIR. 



Use the flask and tube of the experiment of heat expanding water. 

 Have the flask clean and dry. Place the end of the tube under water. 

 Warm the flask with flame of the lamp. As the bubbles of air escape, 

 have the children observe what happens and explain. Cool the flask. 

 Repeat the experiment several times; or better, allow the children to 

 repeat it. 



See how sensitive the air is to even a small amount of heat. Arrange 

 flask and tube so that the mouth of the tube is covered with water, hav- 

 ing driven out enough air so that the water will rise well up in the tube. 

 Place the hand or just one or two fingers on the flask and observe the 

 change of volume. 



Arrange the flask and tube in a horizontal position. Get a little water 

 in about the middle of the tube. It is better to have it colored with ink 

 or some other coloring matter; this will enable its motion to be more 

 plainly seen. Repeat the experiments with this apparatus. It will be 

 very sensitive to small amounts of heat. 



In addition to these experiments, which are to be examined very 

 closely and seen clearly till they are well understood, many other illus- 

 trations of the fact that heat expands air may be devised by both the 

 pupils and yourself. For example, a football or a bladder partly blown 

 up, then heated, fills out by the expansion of the contained air. 



The flask and tube with a paper scale may be used as a thermometer. 

 For this purpose support the flask so that it stands perpendicularly with 

 the mouth of the tube immersed in water, with the water rising a little 

 way up in the tube; or support the whole in a horizontal position, putting 

 a drop of liquid in the tube. In the latter case a drop of water cannot 

 be used, as it will evaporate. Use a small tube and a drop of strong 

 sulphuric acid or a drop of mercury. 



What makes the best thermometer of the substances thus far 

 used? Why? Give time for discussion and comparative experiments. 



As it is desirious, among other things, to use the knowledge of the 

 effects of heat on air to understand currents of air, winds, etc., it might 

 be well to next attack the subject of why heated air rises. Children 

 have generally learned the statement that " heated air rises," but can 



