Page:Popular Science Monthly Volume 21.djvu/230

220 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 criticise 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 repeating, 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 statement, 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 sample of the general tenor of questions. Sometimes some misapprehension on the part of pupils makes me diverge 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 disappeared from the jar. I test the first oxide with red litmus, both before 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 oxide; 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