1. _By the electric current._ As has been indicated in the preparation
of oxygen, water is easily separated into its constituents, hydrogen and
oxygen, by passing an electric current through it under certain
conditions.

2. _By the action of certain metals._ When brought into contact with
certain metals under appropriate conditions, water gives up a portion
or the whole of its hydrogen, its place being taken by the metal. In the
case of a few of the metals this change occurs at ordinary temperatures.
Thus, if a bit of sodium is thrown on water, an action is seen to take
place at once, sufficient heat being generated to melt the sodium, which
runs about on the surface of the water. The change which takes place
consists in the displacement of one half of the hydrogen of the water by
the sodium, and may be represented as follows:

_ _ _ _
| hydrogen | | sodium |
sodium + | hydrogen |(water) = | hydrogen |(sodium hydroxide) + hydrogen
|_oxygen _| |_oxygen _|

The sodium hydroxide formed is a white solid which remains dissolved in
the undecomposed water, and may be obtained by evaporating the solution
to dryness. The hydrogen is evolved as a gas and may be collected by
suitable apparatus.

Other metals, such as magnesium and iron, decompose water rapidly, but
only at higher temperatures. When steam is passed over hot iron, for
example, the iron combines with the oxygen of the steam, thus displacing
the hydrogen. Experiments show that the change may be represented as
follows:

_ _
| hydrogen | _ _ _ _
iron + | hydrogen |(water) = | iron |(iron oxide) + | hydrogen |
|_oxygen _| |_oxygen _| |_hydrogen_|

The iron oxide formed is a reddish-black compound, identical with that
obtained by the combustion of iron in oxygen.

~Directions for preparing hydrogen by the action of steam on
iron.~ The apparatus used in the preparation of hydrogen from
iron and steam is shown in Fig. 9. A porcelain or iron tube
B, about 50 cm. in length and 2 cm. or 3 cm. in diameter, is
partially filled with fine iron wire or tacks and connected as
shown in the figure. The tube B is heated, slowly at first,
until the iron is red-hot. Steam is then conducted through the
tube by boiling the water in the flask A. The hot iron
combines wi