comparison,
it was found that sunshine affected them most; then brilliant acetylene
gas, which was more effective still when tinted yellow by being passed
through yellow glass. The electric arc was not so effective, but the
electric light of the mercury-vapour lamp, though causing little change
at the first, after a few hours' exposure rapidly bleached certain of
the colours, whilst having no effect on others. Coal gas with
incandescent fibre mantle was slightly effective, whilst the coal-gas,
burned direct through an ordinary burner, affected very few of the
colours, even after twenty-four hours' exposure at a distance of three
feet. In all these cases, though the colours were slightly improved by
the stones being kept for a time in the dark, they failed to recover
their original strength, showing permanent loss of colour.
The Silicates. The
chief of these are the garnets, crystallising in the cubic system, and
anhydrous. The garnet is usually in the form of a rhombic dodecahedron,
or as a trisocta-hedron (called also sometimes an icosatetrahedron), or
a mixture of the two, though the stones appear in other cubic forms. In
hardness they vary from 6-1/2 to 8-1/2. They average from 40 to about
42 per cent, of silica, the other ingredients being in fairly constant
and definite proportions. They are vitreous and resinous in their
lustre and of great variety of colour, chiefly amongst reds, purples,
violets, greens, yellows and blacks, according to the colouring matter
present in their mass. There are many varieties which are named in
accordance with one or more of their constituents, the best known being
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