[Illustration: Figure 21.--Medium Pressure Torch Head]

The construction of the mixing chamber and nozzle is very similar to that
of the high pressure torch, the gases entering in the same way and from the
same positions of openings. The pressure of the acetylene is but little
lower than that of the oxygen, and the two gases, meeting at right angles,
form a very intimate mixture at this point of juncture. The mixture in its
proportions of gases depends entirely on the sizes of the oxygen and
acetylene openings into the mixing chamber and on the pressures at which
the gases are admitted. There is a very slight injector action as the fast
moving stream of oxygen tends to draw the acetylene from the side openings
into the chamber, but the operation of the torch does not depend on this
action to any extent.

_Low Pressure Torches._--The low pressure torch (Figure 22) will use
gas from low pressure generators from medium pressure machines or from
tanks in which it has been compressed and dissolved. This type depends for
a perfect mixture of gas upon the principle of the injector just as it is
applied in steam boiler practice.

[Illustration: Figure 22.--Low Pressure Torch with Separate Injector
Nozzle]

The oxygen enters the head at considerable pressure and passes through its
tube to a small jet within the head. The opening of this jet is directly
opposite the end of the opening through the nozzle which forms the mixing
chamber and the path of the gases to the flame. A small distance remains
between the opening from which the oxygen issues and the inner opening into
the mixing passage. The stream of oxygen rushes across this space and