he bottom of the
jar and mixes with the remaining portion of the hydrogen, so that a
mixture of the two soon forms, and a loud explosion results. The
explosion is not dangerous, since the bottom of the jar is open, thus
leaving room for the expansion of the hot gas.

Since air is only one fifth oxygen, the remainder being inert gases, it
may readily be inferred that a mixture of hydrogen with pure oxygen
would be far more explosive than a mixture of hydrogen with air. Such
mixtures should not be made except in small quantities and by
experienced workers.

~Hydrogen does not support combustion.~ While hydrogen is readily
combustible, it is not a supporter of combustion. In other words,
substances will not burn in it. This may be shown by bringing a lighted
candle supported by a stiff wire into a bottle or cylinder of the pure
gas, as shown in Fig. 14. The hydrogen is ignited by the flame of the
candle and burns at the mouth of the bottle, where it comes in contact
with the oxygen in the air. When the candle is thrust up into the gas,
its flame is extinguished on account of the absence of oxygen. If slowly
withdrawn, the candle is relighted as it passes through the layer of
burning hydrogen.

[Illustration: Fig. 14]

[Illustration: Fig. 15]

~Reduction.~ On account of its great affinity for oxygen, hydrogen has the
power of abstracting it from many of its compounds. Thus, if a stream of
hydrogen, dried by passing through the tube B (Fig. 15), filled with
calcium chloride, is conducted through the tube C containing some
copper oxide, heated to a moderate temperature, the hydrogen abstracts
the oxygen from the copper oxide. The change may be represented as
follows:

hydrogen + {copper} {hydrogen}
{oxygen}(copper oxide) = {oxygen }(water) + copper

The water formed collects in the cold portions of the tube C near its
end. In this experiment the copper oxide is said to undergo reduction.
_Reduction may therefore be defined as the process of withdrawing oxygen
from a compound._

~Relation of reduction to oxidation.~ At the same time that the copper
oxide is reduced it is clear that the hydrogen is oxidized, for it
combines with the oxygen given up by the copper oxide. The two processes
are therefore very closely related, and it usually happens that when one
substance is oxidized some other substance is reduced. That substance
which gives up its oxygen is called an _oxidizing age