nstitution,
1901.

=Properties of Vitreous Silica.=--For the convenience of those who are
not familiar with the literature of this subject, I may commence this
chapter with a brief account of the properties and applications of
vitreous silica, as far as they are at present ascertained. Vitreous
silica is less hard than chalcedony, but harder than felspar. Tubes and
rods of it can be cut with a file or with a piece of sharpened and
hardened steel, and can afterwards be broken like similar articles of
glass. Its conducting power is low, and Mr. Boys has shown that fine
fibres of silica insulate remarkably well, even in an atmosphere
saturated with moisture. The insulating qualities of tubes or rods of
large cross sections have not yet been fully tested; one would expect
them to give good results provided that they are kept scrupulously
clean. A silica rod which had been much handled would probably insulate
no better than one of glass in a similar condition. The density of
vitreous silica is very near to that of ordinary amorphous silica. In
the case of a small rod not absolutely free from minute bubbles it was
found to be 2.21.

Vitreous silica is optically inactive, when homogeneous, and is highly
transparent to ultraviolet radiations.

The melting point of vitreous silica cannot be definitely stated. It is
plastic over a considerable range of temperature. Professor Callendar
has succeeded in measuring the rate of contraction of fine rods in
cooling from 1200 deg. to 1500 deg. C., so that its plasticity must be very
slight below the latter temperature. If a platinum wire embedded in a
thick silica tube be heated from without by an oxy-hydrogen flame the
metal may be melted at temperatures at which the silica tube will retain
its form for a moderate length of time, but silica softens to a marked
extent at temperatures a little above the melting point of platinum.

It has been observed by Boys, Callendar, and others that fine rods of
silica, and also the so-called "quartz fibres," are apt to become
brittle after they have been heated to redness. But I have not observed
this defect in the case of more massive objects, such as thick rods or
tubes; and as I have repeatedly observed that mere traces of basic
matter, such as may be conveyed by contact with the hand, seriously
injure the surface of silica, and have found that silica quickly becomes
rotten when it is heated to about 1000 deg. in contact with an infusible
b