MANIPULATION.
17. To Break Glass Tubing.
Experiment 8.--Lay the tubing on a flat surface, and draw a sharp
three-cornered file two or three times at right angles across it
where it is to be broken, till a scratch is made. Take the tube
in the hands, having the two thumbs nearly opposite the scratch,
and the fingers on the other side. Press outward quickly with the
thumbs, and at the same time pull the hands strongly apart, and
the tubing should break squarely at the scratch.
To break large tubing, or cut off bottles, lamp chimneys, etc.,
first make a scratch as before; then heat the handle of a file,
or a blunt iron--in a blast-lamp flame by preference--till it is
red-hot, and at once press it against the scratch till the glass
begins to crack. The fracture can be led in any direction by
keeping the iron just in front of it. Re-heat the iron as often
as necessary.
18. To Make Ignition-Tubes.
Experiment 9.--Hold the glass tubing between the thumb and
forefinger of each hand, resting it against the second finger.
Heat it in the upper flame, slowly at first, then strongly, but
heat only a very small portion in length, and keep it in constant
rotation with the right hand. Hold it steadily, and avoid
twisting it as the glass softens. The yielding is detected by the
yellow flame above the glass and by an uneven pressure on the
hands. Pull it a little as it yields, then heat a part just at
one side of the most softened portion. Rotate constantly without
twisting, and soon it can be separated into two closed tubes. No
thread should be attached; but if there be one, it can be broken
off and the end welded. The bottom can be made more symmetrical
by heating it red-hot, then blowing, gradually, into the open
end, this being inserted in the mouth. The parts should be
annealed by holding above the flame for a short time, to cool
slowly.
For hard glass--Bohemian--or large tubes, the blast-lamp or
blowpipe is needed. In the blast-lamp air is forced out with
illuminating gas. This gives a high degree of heat. Bulbs can be
made in the same way as ignition-tubes, and thistle-tubes are
made by blowing out the end of a heated bulb, and rounding it
with charcoal.
19. To Bend Glass Tubing.
Experiment 10.--Hold the tube in the upper flame. Rotate it so as
to heat all parts equally, and let the flame spread over 3 or 4
cm. in length. When the glass begins to yield, without removing
from the flame slowly bend it as desired. Avoid twisting, and be
sure to have all parts in the same plane; also avoid bending too
quickly, if you would have a well-rounded joint. Anneal each bend
as made. Heated glass of any kind should never be brought in
contact with a cool body. For making O, H, etc., a glass tube --
delivery-tube--50 cm. long should have three bends, as in Figure
6. The pupil should first experiment with short pieces of glass,
10 or 15 cm. long. An ordinary gas flame is the best for bending
glass.
20. To Cut Glass.
Experiment 11.--Lay the glass plate on a flat surface, and draw a
steel glass-cutter--revolving wheel--over it, holding this
against a ruler for a guide, and pressing down hard enough to
scratch the glass. Then break it by holding between the thumb and
fingers, having the thumbs on the side opposite to the scratch,
and pressing them outward while bending the ends of the glass
inward. The break will follow the scratch.
Holes can be bored through glass and bottles with a broken end of
a round file kept wet with a solution of camphor in oil of
turpentine.
21. To Perforate Corks.
Experiment 12.--First make a small hole in the cork with the
pointed handle of a round--rat-tail--file. Have the hole
perpendicular to the surface of the cork. This can be done by
holding the cork in the left hand and pressing against the larger
surface, or upper part, of the cork, with the file in the right
hand. Only a mere opening is made in this way, which must be
enlarged by the other end of the file. A second or third file of
larger size may be employed, according to the size of the hole to
be made, which must be a little smaller than the tube it is to
receive, and perfectly round.