Substances. 129 



Take first a piece of clear glass and get the pupils to 

 describe it. Bring out its transparency, weight, hardness, 

 brittleness, and other obvious properties. Have a piece 

 broken and examine the fracture carefully. It breaks not 

 in smooth surfaces (planes of cleavage), but rather in 

 irregular lines. These fresli edges will also show what 

 appear to be shell-like markings (conchoidal fractures). I 

 have sometimes had pupils bring me stones where this 

 peculiar breaking looked so like a shell that they were taken 

 for fossils. Drop a glass marble upon a flat stone. It bounds 

 back. It is elastic. Place a long strip of glass so that its 

 ends are supported, but the middle is not. Now lay a heavy 

 stone on the middle, and if not too heavy the strip of glass 

 will be seen to sag. Remove the weight. It springs back. 

 It is indeed elastic. 



Scratch various things with a piece of glass. It is hard. 

 Can you file glass ? Try and see. Rub it on emery-paper. 

 What happens ? 



Heat the piece of tubing in an alcohol flame. When red- 

 hot, pull gently on the ends. It is ductile. Tell the children 

 about glass-blowing. Tell them about Venetian glassware. 

 Get them to learn what they can about the manufacture and 

 uses of glass. Distribute your specimens and tell them how 

 the colors are produced. Cobalt makes blue, gold red, 

 arsenic white, iron bottle-green, etc. Give as topics for 

 research cut-glass, glass-blowing, etc. 



Place fragments of glass in a stove and see them fuse and 

 run down like wax. Cut glass with hot iron, also with a 

 wheel or ordinary glass-cutter. 



VII. Soap. 



A small quantity of sweet-oil, a very little baking-soda or 

 potash, and a bar of soap are needed. Let the pupils see 

 the oil and the soda. Then place the oil in a baking-powder 

 box cover and add a strong solution of soda drop by drop. 

 Heat over a lamp or upon the stove. The oil will curdle 

 and form a soap. When cold, it may be examined by the 



