CHEMISTRY IN HIGH-SCHOOLS. 221 



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 car- 

 bon 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 train- 

 ing, 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 wa- 

 ter, 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 oxy- 

 gen 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.* 



* In final review, I employ the topical method of recitation, but this method is too 

 loose for investigation, which must be held down to accuracy, by well-considered ques- 

 tions, taking up one pqjnt at a time. 



