n the generator. If a flame is brought near the
jet before this mixture has all escaped, a violent and very
dangerous explosion results, since the entire apparatus is
filled with the explosive mixture. On the other hand, if the
flame is not applied until all the air has been expelled, the
hydrogen is ignited and burns quietly, since only the small
amount of it which escapes from the jet can come in contact
with the oxygen of the air at any one time. By holding a cold,
dry bell jar or bottle over the flame, in the manner shown in
the figure, the steam formed by the combustion of the hydrogen
is condensed, the water collecting in drops on the sides of the
jar.

~Precautions.~ In order to avoid danger it is absolutely necessary to
prove that the hydrogen is free from air before igniting it. This can be
done by testing small amounts of the escaping gas. A convenient and safe
method of doing this is to fill a test tube with the gas by inverting it
over the jet. The hydrogen, on account of its lightness, collects in the
tube, displacing the air. After holding it over the jet for a few
moments in order that it may be filled with the gas, the tube is gently
brought, mouth downward, to the flame of a burner placed not nearer than
an arm's length from the jet. If the hydrogen is mixed with air a slight
explosion occurs, but if pure it burns quietly in the tube. The
operation is repeated until the gas burns quietly, when the tube is
quickly brought back over the jet for an instant, whereby the escaping
hydrogen is ignited by the flame in the tube.

[Illustration. Fig. 13]

~A mixture of hydrogen and oxygen is explosive.~ That a mixture of
hydrogen and air is explosive may be shown safely as follows: A cork
through which passes a short glass tube about 1 cm. in diameter is
fitted air-tight into the tubule of a bell jar of 2 l. or 3 l. capacity.
(A thick glass bottle with bottom removed may be used.) The tube is
closed with a small rubber stopper and the bell jar filled with
hydrogen, the gas being collected over water. When entirely filled with
the gas the jar is removed from the water and supported by blocks of
wood in order to leave the bottom of the jar open, as shown in Fig. 13.
The stopper is now removed from the tube in the cork, and the hydrogen,
which on account of its lightness escapes from the tube, is at once
lighted. As the hydrogen escapes, the air flows in at t