276. Caution. Do not heat paraffine directly upon the fire or over a burner, unless you watch it constantly. It will burn if its temperature is raised too much. It is better to heat it with steam, as you do glue.
277. Battery Jars. For small cells, use glass tumblers. Ordinary glass fruit jars are good. Even earthen bowls may be used, and for large cells—if you have nothing better—you can use small earthen crocks or jars.
278. Glass Bottles can be cut off so that they will make excellent jars. If you have thin bottles, you can cut them with strong cord. Tie one end of the cord, which should be 5 or 6 feet long, to a door knob or to a solid post. Tie the other end around your body. Make one complete turn of the cord around the bottle where you wish to cut it; draw the cord tight by stepping back, and with both hands draw the bottle back and forth vigorously many times, so that the cord will rub it hard and make it very hot. Do not let the cord move lengthwise upon the bottle. This will make a circle around the bottle that is very hot. Immediately plunge the bottle into cold water, the colder the better. Use ice-water, if you have it. If you produce heat enough, the bottle should crack all the way around very neatly. File off any sharp corners and edges with a wet file.
279. A hot iron can be used with success to cut off a bottle. File a deep groove first, hold the red-hot iron first on one side of file mark and then on the other to start the crack. You can lead the crack wherever you wish by keeping the iron about ⅛ in. ahead of it.
280. A small gas-flame will be much better than a hot iron, and you may easily use it, if you have glass tubing, rubber tubing, etc., in your shop. Draw out the glass so that the gas will burn in a fine needle-like flame about 1 in. long. Keep the point of the flame about ¼ in. ahead of the crack. The glass tube should be held in a rubber tube connected with the gas pipe.