ead glass appears to cool more regularly than soda
glass, and these precautions may be more safely neglected with apparatus
made of lead glass; but not always. At the date of writing I have had
several well-blown joints of thick-walled capillary tube to No. 16 (see
diagram, p. 82), break during cooling, in consequence of circumstances
making it dangerous to heat the neighbourhood of the joint so much as
was necessary.

The black carbonaceous coat formed on hot glass when it is placed in
cotton wool may be removed by wiping with methylated spirit, or, if it
be very closely adherent, by gently rubbing with fine emery, moistened
with the spirit.

Cotton wool is rather dangerously inflammable; it should therefore be
kept out of reach of the blow-pipe flame, and care should be taken that
the glass is not placed in contact with it at a sufficiently high
temperature to cause its ignition.

Another method of annealing is to cover the hot glass with hot sand, and
allow it to cool therein.

As in the case of lead glass, so with soda glass. A thorough
acquaintance with the effect of the various parts of the flame upon it
should be gained before further work is entered upon, for which purpose
an hour or more spent in observing its behaviour in the flame will be
fully repaid by increased success subsequently.

=The Use of Combustion Tube.=--It is often necessary to construct
apparatus of what is known as hard glass or combustion tube. It is
almost as easy to work combustion tube as to deal with lead and soda
glass if the oxy-hydrogen flame be employed.

It is not necessary to set up a special apparatus for this purpose; many
of the ordinary blow-pipes can be used with oxygen instead of with air.
It is only necessary to connect the air-tube of the blow-pipe with a
bottle of compressed oxygen instead of with the bellows. The connecting
tube should not be too wide nor too long, in order to avoid the
accumulation in it, by accident, of large quantities of explosive
mixtures.

Two precautions are necessary in manipulating hard glass in the
oxy-hydrogen flame. The glass must _not_ be overheated. At first one is
very apt to go wrong in this direction. The supply of oxygen must _not_
be too great; a small hissing flame is not what is wanted. If either of
these precautions are neglected most glass will devitrify badly. With a
little care and experience, devitrification can be absolutely avoided.
Ordinary combustion tube can be use