Page:Popular Science Monthly Volume 21.djvu/231

Rh of mercury, and that result tested. Every particle of information which observation can draw from these experiments is carefully elicited by questions such as I have described.

At this point I usually inform them that the red oxide of mercury is sometimes made by heating mercury a long time in contact with air. They commonly by this time consider the evidence of oxygen in the air pretty conclusive. I next lead them to think about the air we breathe: whether it comes from our lungs unchanged; to think of some way of testing whether it contains free oxygen. I have them test the breath with lime-water, discuss the effect of the union of carbon and oxygen, especially the heat.

In all this I tell them very little. I become greatly interested in seeing how much I can get them to do for themselves. I simply try to stimulate and get them on the right track. At this point I usually ask them whether they think the air contains anything besides oxygen, and set them to discussing ways of getting at the other element in air.

Devising experiments is a very important part of chemical training, and, where the pupil sees beforehand some question to settle, he can be made to do it. By rousing him to think, criticising his crude plan, and showing, or making him think of, its defects, it can be done. Pupils will devise the well-known experiment of burning away oxygen from air, but of course they must be told that phosphorus is the best combustible for their purpose. By taking up the various requirements of the experiment separately, they will suggest nearly everything.

But before the experiment is actually carried out, to prevent the confusion which would arise from the vapor of phosphoric anhydride, I have them make a little study of phosphorus. It is examined, burned in oxygen, burned in air, the anhydride noted, its great affinity for water, its behavior to litmus both before and after union with water, its taste noted, etc. After this we use phosphorus to help us study the composition of air. The girls note (approximately) the proportion of oxygen to nitrogen.

We test the air for carbonic acid; discuss the moisture in it, etc.; and then I have them make some study of water. To do this I first put a bit of sodium in a very small cage of wire gauze, and thrust it under a little water. The result is tested, and shown to be the same compound they before knew. When they are satisfied that the oxygen must come from the water, we collect hydrogen and examine it. I have them also note the new method of decomposition. Then we have the proof by synthesis, burning hydrogen and collecting a little of the water.

As we proceed, my pupils begin to think ahead of questions, and their perceptions grow sharper.