e
a means for readily producing very uniform cylindrical threads, but
that the limits of allowable rate of twist are so wide that a small
departure from uniformity of section produces much less inconvenience
than in the case of any other known substance.

Sec. 82. There are three methods generally in use for drawing quartz
fibres, all depending on the fact that quartz when fused is so viscous
that it may be drawn into threads of great length, without these
threads breaking up into drops, or indeed without their showing any
sign of doing so. The surface tension of the melted quartz must,
however, be very considerable, as may be seen by examining the shape
of a drop of the molten material, and this suffices to impress a
rigidly cylindrical form upon the thread, the great viscosity
apparently damping down all oscillation.

The first method is the one originally employed by Mr. Boys. A needle
of quartz is melted somewhere in its length and is then drawn out
rapidly by a light arrow, to which one end of the needle is attached,
and which is projected from a kind of crossbow.

A modification of this method, which the writer has found of service
when very thick threads are required, is to replace the bow and arrow
by a kind of catapult.

The third method, which yields threads of almost unmanageable
fineness, depends on the experimental fact that when a fine point of
quartz is held in a high pressure oxygen gas blow-pipe flame, the
friction of the flame gases suffices to overcome the tendency of the
capillary forces to produce a spherical drop, and actually causes a
fine thread to be projected outwards in the direction of the flame.

Sec. 83. A preliminary operation to any method is the production of a
stick of fused quartz. This is managed as follows. A rock crystal or
quartz pebble is selected and examined. It must be perfectly white,
transparent, and free from dirt. Surface impurity can of course be
got rid of by means of a grindstone. The crystal is placed in a
perfectly clean Stourbridge clay crucible, furnished with a cover, and
heated to bright redness for about an hour in a clean fire or in a
Fletcher's gas furnace. The contents of the crucible are turned out
when sufficiently cool on to a clean brick or bit of slate. It will
be found that the crystal is completely broken up and the fragments
must be examined in case any of them have become contaminated by the
crucible, but this will not have happened if