rocedure, and, as far as may be, to
memorise it. Once having mastered the underlying reason, he can evolve
schemes of manipulation to suit his own particular needs, although, as a
rule, those given in the following pages will be found to embody the
result of many years' experience.

There is a wide choice of apparatus, from a simple mouth-blowpipe and a
candle flame to a power-driven blower and a multiple-jet heating device.
All are useful, and all have their special applications, but, for the
present, we will consider the ordinary types of bellows and blowpipes,
such as one usually finds in a chemical or physical laboratory.

The usual, or Herepath, type of gas blowpipe consists of an outer tube
through which coal gas can be passed and an inner tube through which a
stream of air may be blown. Such a blowpipe is shown in section by Fig.
1. It is desirable to have the three centring screws as shown, in order
to adjust the position of the air jet and obtain a well-shaped flame,
but these screws are sometimes omitted. Fig. 1, _a_ and _b_ show the
effects of defective centring of the air jet, _c_ shows the effect of
dirt or roughness in the inside of the air jet, _d_ shows a satisfactory
flame.

[Illustration: Fig 1]

For many purposes, it is an advantage to have what is sometimes known
as a "quick-change" blowpipe; that is one in which jets of varying size
may be brought into position without stopping the work for more than a
fraction of a second. Such a device is made by Messrs. Letcher, and is
shown by _e_, and in section by _f_ Fig. 1. It is only necessary to
rotate the desired jet into position in order to connect it with both
gas and air supplies. A small bye-pass ignites the gas, and adjustment
of gas and air may be made by a partial rotation of the cylinder which
carries the jets.

For specially heavy work, where it is needed to heat a large mass of
glass, a multiple blowpipe jet of the pattern invented by my father,
Thomas Bolas, as the result of a suggestion derived from a study of the
jet used in Griffin's gas furnace, is of considerable value. This jet
consists of a block of metal in which are drilled seven holes, one being
central and the other six arranged in a close circle around the central
hole. To each of these holes is a communication way leading to the gas
supply, and an air jet is arranged centrally in each. Each hole has also
an extension tube fitted into it, the whole effect being that of seven