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212
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GEMS.
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Emery
is most useful in all arts connected with glass, stone, and metal work.
It can be reduced into exceedingly fine powder, and when used it is
best melted in water, oil, diluted sulphuric acid, or in vinegar. It is
often used spread and glued over a strong, stiff card-paper. It is
impossible to judge at first sight the quality of an emery. Certain
kinds would not do to work stones of medium hardness, being too large
in grain. It is remarkable that these are useful in cutting harder
stones, and this happens because the grains are then crushed by the
hardness of the stone, and reduced to the finest powder.
XCII.
SPUMA-MARINA.
Hydro-sillcate of magnesia found in nodules, in various countries, and bearing the above name, from the German word meerschaum, which signifies froth of the sea, because the German labourers who gather it in the Baltic provinces believed that it was produced by the waters of the ocean.
XCIII.
STALACTITE.
This name is giyen to certain calcareous spars in thy form of cones, pillars, and columns.
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