examine its properties.

[25] Messrs. Baird and Tatlock.

=Preparing non-splintering Silica from Brazil Pebble.=--The best variety
of native Silica is Brazil Pebble, which may be obtained in chips or
larger masses. These should be thoroughly cleaned, heated in boiling
water, and dropped into cold water, the treatment being repeated till
the masses have cracked to such an extent that they may be broken easily
by blows from a clean steel pestle or hammer.

The fragments thus produced must be hand-picked, and those which are not
perfectly free from foreign matter should be rejected. The pure and
transparent pieces must then be heated to a yellow-red heat in a covered
platinum dish in a muffle or reverberatory furnace and quickly plunged
into a deep clean vessel containing clean distilled water; this process
being repeated, if necessary, till the product consists of semi-opaque
friable masses, very much like a white enamel in appearance. After these
have been washed with distilled water, well drained and dried, they may
be brought into the hottest part of an oxy-gas flame safely, or pressed
suddenly against masses of white hot silica without any preliminary
heating, such as is necessary in the case of natural quartz. Quartz
which has not been submitted to the above preparatory process, splinters
on contact with the flame to such an extent that very few would care to
face the trouble and expense of working with so refractory a material.
But after the above treatment, which really gives little trouble, all
the difficulties which hampered the pioneer workers in silica disappear
as if by magic.

=Apparatus.=--Very little special apparatus need be provided for working
with silica, but it is absolutely essential to protect the eyes with
very dark glasses. These should be so dark as to render it a little
difficult to work with them at first. If long spells of work are
undertaken, two pairs of spectacles should be provided, for the glasses
quickly become hot enough to cause great inconvenience and even injury
to the eyes.

Almost any of the available oxy-gas burners may be used, but they vary
considerably in efficiency, and it is economical to obtain a very
efficient burner. The 'blow-through' burners are least satisfactory, and
I have long since abandoned the use of them. Some of the safety
'mixed-gas jets' have an inconvenient trick of burning-back, with sharp
explosions, which are highly disconcerting, if the work be br