nt_, while the
substance which unites with the oxygen is called a _reducing agent_.

~The oxyhydrogen blowpipe.~ This is a form of apparatus used for burning
hydrogen in pure oxygen. As has been previously stated, the flame
produced by the combustion of hydrogen in the air is very hot. It is
evident that if pure oxygen is substituted for air, the temperature
reached will be much higher, since there are no inert gases to absorb
the heat. The oxyhydrogen blowpipe, used to effect this combination,
consists of a small tube placed within a larger one, as shown in Fig.
16.

[Illustration: Fig. 16]

The hydrogen, stored under pressure, generally in steel cylinders, is
first passed through the outer tube and ignited at the open end of the
tube. The oxygen from a similar cylinder is then conducted through the
inner tube, and mixes with the hydrogen at the end of the tube. In order
to produce the maximum heat, the hydrogen and oxygen must be admitted to
the blowpipe in the exact proportion in which they combine, viz., 2
volumes of hydrogen to 1 of oxygen, or by weight, 1 part of hydrogen to
7.94 parts of oxygen. The intensity of the heat may be shown by bringing
into the flame pieces of metal such as iron wire or zinc. These burn
with great brilliancy. Even platinum, having a melting point of 1779 deg.,
may be melted by the heat of the flame.

While the oxyhydrogen flame is intensely hot, it is almost non-luminous.
If directed against some infusible substance like ordinary lime (calcium
oxide), the heat is so intense that the lime becomes incandescent and
glows with a brilliant light. This is sometimes used as a source of
light, under the name of _Drummond_ or _lime light_.

[Illustration: Fig. 17]

~The blast lamp.~ A similar form of apparatus is commonly used in the
laboratory as a source of heat under the name _blast lamp_ (Fig. 17).
This differs from the oxyhydrogen blowpipe only in the size of the
tubes. In place of the hydrogen and oxygen the more accessible coal gas
and air are respectively used. The former is composed largely of a
mixture of free hydrogen and gaseous compounds of carbon and hydrogen.
While the temperature of the flame is not so high as that of the
oxyhydrogen blowpipe, it nevertheless suffices for most chemical
operations carried out in the laboratory.

~Uses of hydrogen.~ On account of its cost, hydrogen is but little used
for commercial purposes. It is sometimes used as a material for the
infla