220 THE POPULAR SCIENCE MONTHLY. 



bodies ? Name some kinds of attraction. Is this force cohesion ? 

 Why not ? Is it adhesion ? Why not ? " I finally tell them the force 

 is called chemical attraction, and I call on them to put into words a 

 clear definition of chemical attraction. As they do so, I simply criti- 

 cise the successive trials they make, until the definition is correct in 

 matter and form ; and then, after making them repeat it several times 

 in concert, I write it on the blackboard. 



By similar questions, which I will not weary the reader by repeat- 

 ing, I make them draw from the same experiment facts and definitions 

 about elements, simple substances, compounds, oxides, decomposition, 

 etc., etc. 



In my life, I have taught a great deal of Latin and English, but I 

 know no such language-lesson as is given when a class, under the fire 

 of a skillful teacher's criticism, slowly perfects a clear logical state- 

 ment, or definition, for which they have gained the material by using 

 their own senses and reason. My pupils keep note-books, and at every 

 lesson bring me, neatly written out, the substance of the previous lesson. 



I have given above only a samj)le of the general tenor of questions. 

 Sometimes some misapprehension on the part of pupils makes me di- 

 verge widely to bring them back on the track. 



I endeavor to make the subject as practical as possible by having 

 pupils study the chemistry of common operations. After the above 

 experiment, I usually introduce the subject of air by asking : " Why 

 did the taper at last fail to light again when dipped into the jar ? 

 What had become of the oxygen in the jar? Did the remains of the 

 burned paper look like the remains when paper is burned in the air ? " 



After this I take a bit of sodium and burn it in oxygen. I also 

 oxidize some in the air. I show, by testing, that the oxygen has dis- 

 appeared from the jar. I test the first oxide with red litmus, both be- 

 fore and after dissolving in water. I let the pupils taste a little of a 

 very dilute solution. I then ask questions about sodium just as I did 

 about oxygen and mercury. I ask what has become of the oxygen 

 and the sodium ; what unites them ; what the force is called ; draw 

 them on to classify the result of the union as a compound and an ox- 

 ide ; draw them on to note the properties of the compound. Then I 

 have them test the oxide formed in the air just as the other was 

 tested. I ask : " Is this a compound ? Do you think you know either 

 of its elements ? Of which one are you sure ? Do you think you 

 know the other element ? Why do you think it is oxygen ? Where 

 did the oxygen come from ? What other reason have you for thinking 

 the air contains oxygen ? " 



I wait for further experiments before indorsing their partly formed 

 conclusion about oxygen in the air. We next make some study of 

 carbon by burning coal in oxygen. I have them test the result with 

 lime-water. We burn coal in a receiver of air, and test this result also. 

 I have some powdered charcoal heated in contact with the red oxide 



