oo great shrinkage occurs, the place may be blown out gently after
reheating. Thus the wire is sealed through the wall of the tube without
changing the thickness of the latter, and consequently without
developing undue stresses at that point. Such a joint must of course be
carefully reheated and annealed. With fine platinum wire there is very
little risk of the tube cracking if care is taken to avoid formation of
any lump and to reheat the whole circumference of the tube at that
point.

Any glass adhering to the end of the platinum wire, where the scrap of
glass was sealed on for a handle, may be removed when the glass has
cooled by crushing it carefully with a pair of pliers.

SEALING VACUUM TUBES

Tubes which have been evacuated usually are sealed off while they are
still connected to the vacuum pump. The connection should be through a
small, rather thick-walled tube. When this is to be sealed, it is slowly
heated toward the softening point. As the glass just begins to soften,
the air-pressure will force it in, and care must be taken that the
softening is uniform over the whole circumference of the tube. As the
shrinking goes on, the tube is gently drawn out to make a thick-walled
cone at that place, and the end is drawn off as soon as the tube is
sealed. The principal point to be guarded is the thickness of the walls
of the cone, and uniform heating. A thin place or a hot place will give
way under the air-pressure and be sucked into the tube.

CLOSED TUBES FOR HEATING UNDER PRESSURE

(_Carius method for determination of the halogens and sulphur._) In this
case the tubing used must have thick walls (usually about 3/32 inch) to
withstand the pressure. Its external diameter is usually about 3/4 inch.
One length will usually make two tubes of standard length for the cannon
furnace. Especial care must be taken in heating and cooling it on
account of the thick walls. A length is gradually warmed in the center,
finally heated at that point until soft, drawn out, cut apart and
annealed. Taking one of the pieces, the cone is carefully heated and
shrunk, as in Exercise 4, until its walls are as thick as those of the
main tube. A flame with a little tinge of yellow should be used for this
operation to prevent devitrification (page 2), as the thick glass
shrinks slowly. The tail is now drawn off and the whole end heated and
gently blown several times to make a rounded end, like a test-tube, with
walls as thick as those