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 with the oxygen in the steam, setting free the hydrogen, which is collected over water. The gas which first passes over is mixed with the air previously contained in the flask and tube, and is allowed to escape, _since a mixture of hydrogen with oxygen or air explodes violently when brought in contact with a flame_. It is evident that the flask A must be disconnected from the tube before the heat is withdrawn.