ILLUSTRATIVE MATERIAL 41 



expansion appears. Be sure that the children see it and 

 note the cause. Then there will be no question of their con- 

 cluding that heat causes expansion. 



Why not let it go with one experiment ? Because just one 

 case is not sufficient warrant for a broad generahzation. 

 Several experiments, all agreeing and showing the same 

 thing, will be more convincing than only one. To be actually 

 positive that heat causes expansion would require us to with- 

 hold judgment until we had made an infinite number of ex- 

 periments or observations on all the cases that were possible. 

 But we are satisfied to make a fair number of tests, and then 

 to reason by analogy and apply the law to all cases. In the 

 above case — expansion caused by heat — if the pupil is led 

 far enough in his studies, he will discover some anomalous 

 cases or exceptions to the general rule. He finds, for ex- 

 ample, that water near its freezing point actually contracts 

 on being warmed, and expands on being cooled. So it will 

 be well to preserve an attitude of open-mindedness in science, 

 as in all other things, for our preconceived notions may not 

 be quite right. 



After the general principle of a series of experiments has 

 been reasoned out, the pupils should then find applications 

 in common life. Besides being an excellent drill in the 

 principle studied, this will connect the school-room lesson 

 with things of daily life. 



Experiments require unusual care in their preparation 

 and must be well chosen. The teacher ought never to try an 

 experiment in class which she has not done before, as it may 

 not "work," and she will then lay herself open to criticism 

 from her pupils, who will rightly judge her as either careless in 

 preparation or as ignorant of her subject. The omission 



