ndow-Glass; Joining Rod, Feet and
Supports--Gripping Devices for use in Corrosive
Solutions--The Building Up of Special Forms from Solid
Glass.

Perhaps the most common need of the glass-blower whose work is connected
with that of the laboratory is for a sealed tube; and the sealing of a
tube is an excellent preliminary exercise in glass-blowing.

We will assume that the student has adjusted the blowpipe to give a
flame similar to that shown in _d_, Fig. 1, and that he has learned to
maintain a steady blast of air with the bellows; further, we will assume
that the tube he wishes to seal is of moderate size, say not more than
half an inch in diameter and with walls of from one-tenth to one-fifth
of an inch thick.

[Illustration: Fig. 4]

A convenient length of tube for the first trial is about one foot; this
should be cut off from the longer piece, in which it is usually
supplied, as follows:--lay the tube on a flat surface and make a deep
cut with the edge of a file. Do not "saw" the file to and fro over the
glass. If the file edge has been ground as shown in _a_, Fig. 3, such a
procedure will be quite unnecessary and only involve undue wear; one
movement with sufficient pressure to make the file "bite" will give a
deep cut. Now rotate the tube through about one-eighth of a turn and
make another cut in continuation of the first. Take the tube in the
hands, as shown in _a_, Fig. 4, and apply pressure with the thumbs, at
the same time straining at the ends. The tube should break easily. If it
does not, do not strain too hard, as it may shatter and cause serious
injuries to the hands, but repeat the operation with the file and so
deepen the original cuts. In holding a tube for breaking, it is
important to place the hands as shown in sketch, as this method is least
likely to cause shattering and also minimises the risk of injury even if
the tube should shatter. To cut a large tube, or one having very thick
walls, it is better to avoid straining altogether and to break by
applying a small bead of intensely heated glass to the file cut. If the
walls are very thin, a glass-blower's knife should be used instead of a
file. The tube and glass-blower's knife should be held in the hand, and
the tube rotated against the edge of the knife; this will not produce a
deep cut, but is less likely to break the tube. A bead of hot glass
should be used to complete the work.

The next operation is to heat the glass tube