Whenever hydrogen is prepared it should be allowed to escape from the generating vessel for a few minutes before any flame is applied, in order that the atmospheric air may be expelled. The most serious accidents have occurred from carelessness in this respect, as a mixture of hydrogen and air is explosive, and the more dangerous when it takes fire in any closed glass bottle.
Second Experiment.
If a piece of potassium is confined in a little coarse wire gauze cage, attached to a rod, and thrust under a small jar full of water, placed on the shelf of the pneumatic trough, hydrogen gas is produced with great rapidity, and is received into the gas jar. The bit of potassium being surrounded with water, is kept cool, whilst the hydrogen escaping under the water is not of course burnt away, as it is whenever the metal is thrown on the surface of water.
Third Experiment.
Across a small iron table-furnace is placed about eighteen inches of 1-inch gas-pipe containing iron borings, the whole being red-hot; and attached to one end is a pipe conveying steam from a boiler, or flask, or retort, whilst another pipe is fitted to the opposite end, and passes to the pneumatic trough. Directly the steam passes over the red hot iron borings it is deprived of oxygen, which remains with the iron, forming the rust or oxide of iron, whilst the hydrogen, called in this case water gas, escapes with great rapidity. When steam is passed over red-hot charcoal, hydrogen is also produced with carbonic oxide gas, and this in fact is the ordinary process of making water gas, which being purified is afterwards saturated with some volatile hydrocarbon and burnt. At first sight, such a mode of making gas would be thought extremely profitable, and in spite of the numerous failures the discovery (so called) of water gas is reproduced as a sort of chronic wonder; but experience and practice have clearly demonstrated that water gas is a fallacy, and as long as we can get coal it is not worth while going through the round-about processes of first burning coal to produce steam; secondly, of burning coal to heat charcoal, over which the steam is passed to be converted into gas, which has then to be purified and saturated with a cheap hydrocarbon obtained from coal or mineral naphtha; whilst ordinary coal gas is obtained at once by heating coal in iron retorts. (Fig. 111.)
Thus, by the metals zinc, tin, potassium, red-hot iron (and we might add several others), the oxygen of water is removed and hydrogen gas liberated.
Fig. 111.
a. Flask containing water, and producing steam, which passes to the iron tube, b b, containing the iron borings heated red hot in the charcoal stove c. The hydrogen passes to the jar d, standing on the shelf of the pneumatic trough.