Page:Popular Science Monthly Volume 21.djvu/229

Rh chemistry. But it is not all that is needed. Mental activity is the important thing.

I will illustrate the plan which has proved best in my own experience, merely saying, first, that even where pupils have studied physics, I have a preliminary drill to make sure the girls are quite clear about the forms of matter, the properties of liquids and gases, attractive and repulsive forces. I do not say a word to them about chemical attraction, because I wish them to study that out for themselves. Sometimes they have already learned by heart a definition from some book on physics; but, as Thomas Carlyle would say, "by the blessing of Heaven they have generally forgotten it."

I think the best experiment to begin with is the evolution of oxygen gas by heating the red oxide of mercury. This is not the easiest or the most convenient way of obtaining oxygen, but it is much the simplest process. I do not usually tell my pupils anything—not even what the substance is which they are to make; but they know I shall question them closely about what they have seen. When the experiment is concluded—the gas tested with a glowing taper; the residue of mercury examined; and a little of the red oxide put into water and stirred, to show that it will not dissolve—I usually ask questions about as follows:

"When the jar was lifted from the water to the shelf, why did not the water fall out? Why did the water afterward sink in the jar? Did you see anything in the jar as the water fell? Why do you think there was anything in the upper part of the jar? What form of matter was it? How do you know that it was not air? How was it like air? How was it unlike air? Had it color or smell? Did it burn? What was it burned? Did the gas have anything to do with the burning?" I then tell them the gas is called oxygen; and I write on my blackboard the name and symbol, with a list of the properties which they have just discovered.

I then proceed to ask: "What remained in the ignition tube at the close of the experiment? What form of matter was it? Did you ever before see anything with that shiny luster? What class of bodies have it?" "Then I tell those who do not already know that it is mercury, and I give the symbol, Hg.

I then say: "Where were the gas and the liquid when that red powder was placed in the tube? What became of the powder? Did it take any force to separate the gas and liquid which you say formed the powder? What was the force? What sort of a force is heat? Do you suppose any force held the oxygen and mercury together? Do you know a general name given to forces which unite