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 tumeric 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 coloured alumn crystals from breaking, or losing their colour,
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.