g one should also be frequently read over, as many of
the points discussed will not be understood at first and many of the
manipulations described will not be necessary in the simpler exercises.

CHAPTER II

GENERAL OPERATIONS

=Cutting the Glass.=--For this purpose a "glass-knife" is preferred to a
file, if the glass is cold: if it is hot a file must always be used, and
its edge slightly moistened to prevent drawing the temper. The
glass-knife is simply a flat piece of hard steel, with the edges ground
sharp on an emery wheel. The bevel of the edge should be from 30 to 60
degrees. An old flat file can easily be ground into a suitable knife.
The glass-knife makes a narrower scratch than the file but appears more
likely to start the minute crack which is to cause the tube to break at
that point, and the break is more likely to give a good square end. The
scratch should be made by passing part of the knife or file once across
the glass, never by "sawing" the tool back and forth. This latter
procedure dulls the tool very quickly.

In breaking a piece of glass tubing, many persons forget that it is
necessary to _pull_ the ends apart, as well as to bend the tube very
_slightly_ in such a direction as to open up the minute crack started in
the scratch. Care in breaking the tube is essential, as it is impossible
to do as good work with uneven ends as with square ones.

When tubing of large diameter or thin wall is to be cut, it is often
better not to attempt to break it in the usual way, but to heat a very
small globule of glass (1/16 to 1/8 inch diameter) to red heat, and
touch it to the scratch. This will usually start the crack around the
tube; if it has not proceeded far enough, or has not gone in the
desired direction, it may be led along with a hot point of glass. This
is put a little beyond the end of the crack, and as the latter grows out
toward it, moved along the path where the crack is desired. This point
of glass is also very useful in breaking off very short ends of tubes,
where there is not room to get a firm enough hold and sufficient
leverage to break the tube in the ordinary way, and for breaking tubes
attached to large or heavy objects, which would be likely to make
trouble if treated in the ordinary way.

Another way of cutting large tubing, especially if it has rather thick
walls, is to make a scratch in the usual way, and then turn on the
smallest and sharpest possible flame of the blast lamp