acetylene tube under the influence of the high pressure oxygen some form of
back flash preventer is usually incorporated in the torch at or near the
point at which the acetylene enters. This preventer takes the form of some
porous and heat absorbing material, such as aluminum shavings, contained in
a small cavity through which the gas passes on its way to the head.

_High Pressure Torches._--Torches are divided into the same classes as
are the generators; that is, high pressure, medium pressure and low
pressure. As mentioned before, the medium pressure is usually called the
high pressure, because there are very few true high pressure systems in
use, and comparatively speaking the medium pressure type is one of high
pressure.

[Illustration: Figure 20.--High Pressure Torch Head]

With a true high pressure torch (Figure 20) the gases are used at very
nearly equal heads so that the mixing before ignition is a simple matter.
This type admits the oxygen at the inner end of a straight passage leading
to the tip of the nozzle. The acetylene comes into this same passage from
openings at one side and near the inner end. The difference in direction of
the two gases as they enter the passage assists in making a homogeneous
mixture. The construction of this nozzle is perfectly simple and is easily
understood. The true high pressure torch nozzle is only suited for use with
compressed and dissolved acetylene, no other gas being at a sufficient
pressure to make the action necessary in mixing the gases.

_Medium Pressure Torches._--The medium pressure (usually called high
pressure) torch (Figure 21) uses acetylene from a medium pressure generator
or from tanks of compressed gas, but will not take the acetylene from low
pressure generators.