r most purposes, in the case of thin, blowpipe-made or
lamp-blown glass apparatus, it is sufficient to cool slowly by rotating
the finished article over a smoky flame and setting it aside in a place
free from draughts, and where the hot glass will not come in contact
with anything.

Simple bulbs and joints do not even need this smoking; but thick
articles, and especially those that are to be subjected to the stress of
grinding, need more prolonged annealing in a special oven.

_Use of Lead-Glass._--When lead-glass is to be used, the blowpipe flame
should be in good adjustment and the glass should not be allowed to
approach so near to the blue cone as to be blackened. Slight blackening
may often be removed by heating the glass in the extreme end of the
flame.

Lead-glass articles tend to be rather more stable than similar articles
of soda-glass.

_Combustion-Glass._--This may be worked more easily if a small
percentage of oxygen is introduced into the air with which the blowpipe
flame is produced. If the air is replaced entirely by oxygen there is a
risk of damaging the blowpipe jet, unless a special blowpipe is
employed.

_Internal Seal._--There are two ways of making these, one, in which the
inner portion of the tube is fused on to the inside of the bulb or tube
through which it is to pass, an opening is made by bursting and the
outer tube is joined on. This is a quick and in some ways more
satisfactory method than the other, in which there is no separate inner
piece.

_Rubber Blowing Tube._--In complicated work it is often convenient to
use a thin rubber blowing-tube which is connected with the work either
by a cork and piece of glass tubing or by fitting over a drawn-out end.
The use of such a blowing-tube avoids the inconvenience of raising the
work to the mouth when internal air-pressure is required. One end of the
rubber tube is retained in the mouth during work.

_General Notes._--A large amount of glass-blowing is spoiled through
carelessness in arranging the work beforehand. The student should have
every detail of his manipulation clearly in mind before he commences the
work; he should not trust to evolving the method during the actual
manipulation.

Undue haste is another fruitful source of failure. Practically every
operation in glass-blowing can be carried out in a perfectly leisurely
manner, and it is better to err rather on the side of deliberation than
on the side of haste.

If, as will doubt