. After a little experience the adoption of the right-sized
flame for a given purpose, and the perception of the best condition of
glass for blowing it, become almost automatic.

I may add that glass which is to be bent needs to be much less heated
than glass which is to be blown.

=Annealing.=--If apparatus, the glass of which is very thin and of
uniform substance, be heated, on removal from the source of heat it will
cool equally throughout, and therefore may often be heated and cooled
without any special precautions. If the glass be thick, and especially
if it be of unequal thickness in various parts, the thinner portions
will cool more quickly than those which are more massive; this will
result in the production of tension between the thicker and thinner
parts in consequence of inequality in the rates of contraction, and
fractures will occur either spontaneously or upon any sudden shock.
Thus, if a hot tube be touched with cold or wet iron, or slightly
scratched with a cold file, the inequality of the rate of cooling is
great, and it breaks at once. It is therefore necessary to secure that
hot glass shall cool as regularly as possible. And this is particularly
important in the case of articles made of soda glass. Some glass-blowers
content themselves with permitting the glass to cool gradually in a
smoky flame till it is covered with carbon, and then leave it to cool
upon the table. But under this treatment many joints made of soda glass
which are not quite uniform in substance, but otherwise serviceable,
will break down. In glass-works the annealing is done in ovens so
arranged that the glass enters at the hottest end of the oven where it
is uniformly heated to a temperature not much below that at which it
becomes viscous, and slowly passed through the cooler parts of the
chamber so that it emerges cold at the other end. This method of
annealing is not practicable in a small laboratory. But fortunately very
good results can be obtained by the following simple device, viz.:--

By wrapping the hot apparatus that is to be annealed closely in cotton
wool, and leaving it there till quite cold. The glass should be wrapped
up immediately after it is blown into its final shape, as soon as it is
no longer soft enough to give way under slight pressure. And it should
be heated as uniformly as possible, not only at the joint, but also
about the parts adjacent to the joint, at the moment of surrounding it
with the cotton. L