Dissolve alum in hot water until no more can be dissolved in it; place in it a smooth glass rod and a stick of the same size. Next day the stick will be found covered with crystals, but the glass rod will be free from them. In this case the crystals cling to the rough surface of the stick, but have no hold upon the smooth surface of the glass rod.

But if the rod be roughened with a file at certain intervals, and then placed in the alum and water, the crystals will adhere to the rough surfaces, and leave the smooth bright and clear.

Tie some threads of lamp-cotton irregularly around a copper wire or glass rod. Place it in a hot solution of blue vitriol, strong as above, and the threads will be covered with beautiful blue crystals, while the glass rod will be bare.

Bore a hole through a piece of coke, and suspend it by a string from a stick placed across a hot solution of alum. It will float. But as it becomes loaded with crystals it will sink in the solution according to the length of the string. Gas-coke has mostly a smooth, shining, and almost metallic surface, which the crystals will avoid, while they will cling only to the most irregular and porous parts.

If powdered turmeric be added to the hot solution of alum the crystals will be of a bright yellow. Litmus will cause them to be of a bright red. Logwood will yield purple; and common writing ink, black. And the more muddy the solution the finer will be the crystals.

To keep colored alum crystals from breaking or losing their color, place them under a glass shade with a saucer of water.

This will preserve the atmosphere moist, and prevent the crystals getting too dry.

If crystals be formed on wire they will be liable to break off, from the expansion and contraction of the wire by changes of temperature.