Iron and hydrogen are, as mentioned above, about equally combustible: in fact it depends upon the degree of heat, which is the more combustible. If the iron be bright red, it decomposes water, taking away the oxygen; but if it be only dull red, then hydrogen is the more combustible; and if there be a compound of oxygen and iron ready formed (oxide of iron, rust), the hydrogen will decompose it, and water being formed, the iron will be set free. If, therefore, a gun barrel be laid across a fire, and heated to bright redness, and a little water be poured into it at one end by means of a tun-dish with a stop-cock soldered to it, hydrogen gas will issue from the other end, and may be burned, or collected for various purposes.
Hydrogen gas may be prepared more easily by other processes, which do not show, however, so clearly the fact of its being derived from the decomposition of the water. The property which iron acquires at a bright red heat may be given to it without any heat, by means of some oil of vitriol (called in the language of chemists, sulphuric acid). Iron quite cold will decompose water, if the water be previously mixed with some sulphuric acid. The oxygen goes to the iron, which dissolves, and the liquor contains green copperas. The metal zinc, which is now so very much used in the arts, may also be employed with sulphuric acid and water to decompose water, and it gives a purer hydrogen gas than iron, the latter metal containing always a little charcoal, which mixes with the hydrogen and contaminates it.
In all of these processes, although the water is decomposed, yet we obtain only one of its elements; the other, the oxygen, remaining combined with the iron, the charcoal, or the zinc. We may, however, produce the separation of water into its elements, so as to exhibit both. This is done by passing a current of electricity from the apparatus termed the galvanic battery, through the water. One of the grandest and most fruitful discoveries ever made in chemistry was that by Sir Humphry Davy, who proved that electricity possesses the power of separating compound substances into their elements; and by that means he succeeded in decomposing numerous bodies which had resisted all processes known before that time, and obtained new substances of a simple nature, and of most curious and important properties. To decompose water by means of electricity, the wires from the galvanic battery are made to dip into a little cup of water, and over each wire there is hung a bell-shaped vessel, inverted, full of water. When the current passes, pure oxygen gas is disengaged from one wire, and pure hydrogen gas is liberated at the other, and being received as the bubbles rise in the bell-glasses, the gases are collected for use.