fully
investigated, but Mr. H. G. Lacell has this subject in hand. If it
behaves like the other forms of anhydrous silica it will withstand the
action of all acids except hydrofluoric acid. It is, of course, very
readily acted upon by solutions of alkalis and alkaline salts.

As regards the use of silica in experiments with gases, it must be
remarked that vitreous silica, like platinum, is slightly permeable to
hydrogen when strongly heated. One consequence of this is that traces of
moisture are almost always to be found inside recently-made silica tubes
and bulbs, however carefully we may have dried the air forced into them
during the process of construction. Owing to the very low coefficient of
expansion of silica, it is not possible to seal platinum wires into
silica tubes. Nor can platinum be cemented into the silica by means of
arsenic enamel, nor by any of the softer glasses used for such purposes.
I have come near to success by using kaolin, but the results with this
material do not afford a real solution of the problem, though they may
perhaps point to a hopeful line of attack. Possibly platinum wires might
be soldered into the tubes (see _Laboratory Arts_, R. Threlfall), but
this also is uncertain.

The process of preparing silica tubes, etc., from Lumps of Brazil
Crystal may be described conveniently under the following headings. I
describe the various processes fully in these pages, as those who are
interested in the matter will probably wish to try every part of the
process in the first instance. But I may say that in practice I think
almost every one will find it advantageous to start with purchased
silica tubes, just as a glass-worker starts with a supply of purchased
glass tubes. The manufacturer can obtain his oxygen at a lower price
than the retail purchaser, and a workman who gives much time to such
work can turn out silica tube so much more quickly than an amateur, that
I think it will be found that both time and money can be saved by
purchasing the tube. At the same time the beginner will find it worth
while to learn and practise each stage of the process at first, as every
part of the work described may be useful in the production of finished
apparatus from silica tubes.

This being so, I am glad to be able to add that a leading firm of
dealers in apparatus[25] has commenced making silica goods on a
commercial scale, so that the new material is now available for all
those who need it or wish to