One of the most important points to be observed, however, is the part of the jet of flame to be used.
If
the outside flame is used, metallic bodies become oxidized; if the
inner flame, which is of a bluish-red colour, and which is the hottest
portion of it, the mineral becomes fused.
Experiments are usually tried on small quantities of the gem, ground to powder, and held in a small pla-tina cup.
Some gems are affected by acids. The opal is affected by potash; the
turquoise, garnet, chrysolite, and tourmaline by acids. Some are
unassailable by any chemical substance, particularly the diamond, the
corun-dums, and spinel.
