without bending. It is more costly than iron but also much more
durable.

When used in portable work and for high temperatures, pure nickel
tubes are sometimes used. There is also a special metal tube made
for use in cyanide. This metal withstands the intense penetrating
characteristics of cyanide. It lasts from six to ten months as
against a few days for the iron tube.

The inner tubes of refractory materials, also vary according to
the purposes for which they are to be used. They are as follows:

MARQUARDT MASS TUBES for temperatures up to 3,000 deg.F., but they will
not stand sudden changes in temperature, such as in contact with
intermittent flames, without an extra outer covering of chamotte,
fireclay or carborundum.

[Illustration: FIG. 134.--Other styles of bent mounting.]

FUSED SILICA TUBES for continuous temperatures up to 1,800 deg.F. and
intermittently up to 2,400 deg.F. The expansion at various temperatures
is very small, which makes them of value for portable work. They
also resist most acids.

CHAMOTTE TUBES are useful up to 2,800 deg.F. and are mechanically strong.
They have a small expansion and resist temperature changes well,
which makes them good as outside protectors for more fragile tubes.
They cannot be used in molten metals, or baths of any kind nor
in gases of an alkaline nature. They are used mainly to protect
a Marquardt mass or silica tube.

CARBORUNDUM TUBES are also used as outside protection to other
tubes. They stand sudden changes of temperature well and resist
all gases except chlorine, above 1,750 deg.F. Especially useful in
protecting other tubes against molten aluminum, brass, copper and
similar metals.

CLAY TUBES are sometimes used in large annealing furnaces where they
are cemented into place, forming a sort of well for the insertion of
the thermo-couple. They are also used with portable thermo-couples
for obtaining the temperatures of molten iron and steel in ladles.
Used in this way they are naturally short-lived, but seem the best
for this purpose.

[Illustration: FIG. 135.--Straight thermocouple and guard.]

CORUNDITE TUBES are used as an outer protection for both the Marquardt
mass and the silica tubes for kilns and for glass furnaces. Graphite
tubes are also used in some cases for outer protections.

CALORIZED TUBES are wrought-iron pipe treated with aluminum vapor
which often doubles or even triples the life of the tube at high
temperature.

These tubes com