a point about a quarter of an inch from the
end of the crack and in whatever direction it has to be led. The crack
will now extend towards the source of heat, which should be moved
farther away as the crack advances. In this manner a crack may be caused
to take any desired path and can be led round a large bulb.

_Cutting Glass with the Diamond._--Slips of window-glass can be used in
place of glass rod for some purposes, and as cutting them involves the
use of the glaziers' diamond or a wheel-cutter, they may well be
mentioned under this heading.

In cutting a sheet of glass with the diamond, one needs a flat surface
on which to rest the glass, and a rule against which to guide the
diamond. The diamond should be held in an almost vertical position, and
drawn over the surface of the glass with slight pressure. While this is
being done the angle of the diamond should be changed by bringing the
top of the handle forward until the sound changes from one of scratching
to a clear singing note. When this happens the diamond is cutting. A few
trials will teach the student the correct angle for the diamond with
which he works, and the glass, if properly cut, will break easily. If
the cut fails it is better to turn the glass over and make a
corresponding cut on the other side rather than make any attempt to
improve the original cut. The diamond is seldom used for cutting small
glass tubes.

The use of the wheel-cutter calls for no special mention as it will cut
at any angle, although the pressure required is somewhat greater than
that needed by most diamonds.

CHAPTER III

Internal Seals, Air-Traps, Spray Arresters,
Filter-Pumps--Sprays, Condensers; Plain, Double Surface, and
Spherical--Soxhlet Tubes and Fat Extraction
Apparatus--Vacuum Tubes, Electrode Work, Enclosed
Thermometers, Alarm Thermometers, Recording Thermometers,
"Spinning" Glass.

_Internal Seals._--It is convenient to class those cases in which a
glass tube passes through the wall of another tube or bulb under the
heading of "Internal Seals." These are met with in barometers, spray
arresters, and filter pumps, in condensers and some forms of vacuum
tube. The two principal methods of making such seals will be considered
first and their special application afterwards.

_An Air Trap on a Barometer Tube._--This involves the use of the first
method, and is perhaps the simplest example that can be given. Fig. 9,
_a_, _a1_ an