hydrogen will be ignited by
the flame. It is evident, from this experiment, that in order to retain
the gas in an open bottle the bottle must be placed mouth downward.

[Illustration Fig. 11]

Hydrogen is far more difficult to liquefy than any other gas, with the
exception of helium, a rare element recently found to exist in the
atmosphere. The English scientist Dewar, however, in 1898 succeeded not
only in obtaining hydrogen in liquid state but also as a solid. Liquid
hydrogen is colorless and has a density of only 0.07. Its boiling point
under atmospheric pressure is -252 deg.. Under diminished pressure the
temperature has been reduced to -262 deg.. The solubility of hydrogen in
water is very slight, being still less than that of oxygen.

Pure hydrogen produces no injurious results when inhaled. Of course one
could not live in an atmosphere of the gas, since oxygen is essential to
respiration.

~Chemical properties.~ At ordinary temperatures hydrogen is not an active
element. A mixture of hydrogen and chlorine, however, will combine with
explosive violence at ordinary temperature if exposed to the sunlight.
The union can be brought about also by heating. The product formed in
either case is hydrochloric acid. Under suitable conditions hydrogen
combines with nitrogen to form ammonia, and with sulphur to form the
foul-smelling gas, hydrogen sulphide. The affinity of hydrogen for
oxygen is so great that a mixture of hydrogen and oxygen or hydrogen
and air explodes with great violence when heated to the kindling
temperature (about 612 deg.). Nevertheless under proper conditions hydrogen
may be made to burn quietly in either oxygen or air. The resulting
hydrogen flame is almost colorless and is very hot. The combustion of
the hydrogen is, of course, due to its union with oxygen. The product of
the combustion is therefore a compound of hydrogen and oxygen. That this
compound is water may be shown easily by experiment.

[Illustration Fig. 12]

~Directions for burning hydrogen in air.~ The combustion of
hydrogen in air may be carried out safely as follows: The
hydrogen is generated in the bottle A (Fig. 12), is dried by
conducting it through the tube X, filled with some substance
(generally calcium chloride) which has a great attraction for
moisture, and escapes through the tube T, the end of which is
drawn out to a jet. The hydrogen first liberated mixes with the
air contained i