This is an elementary gas and the lightest substance known. It is colourless, odourless, and non-poisonous, although, as ordinarily prepared, it frequently contains traces of disagreeably smelling or of poisonous impurities.

Hydrogen is obtained by the decomposition of water in various ways. It is usually prepared by the action of zinc or iron on a solution of hydrochloric or sulphuric acid. All metals which readily decompose water when heated readily furnish hydrogen on a similar treatment. Many other acids may be used, but none cut more readily. In all cases the action consists in the displacement of the hydrogen of the acid by the metal employed, and if the acid is not one which can enter into reaction with the displaced nitrogen, the latter is evolved as gas.

On the large scale mostly pure hydrogen may be prepared by passing steam over charcoal or coke heated to dull redness. If the temperature be kept sufficiently low, hydrogen and carbon dioxide are the products (C + 2 H20 = 2 H2 + CO2), and the latter may be removed by causing the gas to traverse a vessel filled with slaked lime.

Hydrogen is also obtained pure by the electrolytic decomposition of water, as described under the heading "Oxygen' on page 27.

The liquefaction of gaseous hydrogen is an achievement the more remarkable as it was the result of the simultaneous but entirely independent labours of two distinguished physicists, Cailletet of Chatillon-sur-Seine and Raoul Pictet of Geneva - by the former on the 30th December, 1877, and by the latter on the 10th January, 1878.

When inhaled, hydrogen imparts a peculiar squeaking tone to the voice, due to the extreme tenuity of the gas; small animals, when put into it, die instantly. Hydrogen, however, is not directly poisonous, but may cause death by preventing access of oxygen to the lungs.

Hydrogen when mixed with air or oxygen is explosive; the loudest explosion is obtained by mixing together two volumes of hydrogen and one volume of oxygen.

Knowledge and Scientific News, in its October number, 1908, refers to a new method of preparing hydrogen as described by Mauricheau-Bavpre in the current number of Cowptes Rendus. A coarse powder is first prepared by the interaction of a small quantity of mercuric chloride and potassium cyanide with fine aluminium filings, and on adding water to the resulting compound in the proportion of one litre per kg., hydrogen is slowly evolved, the oxidation of one kg. being complete in about two hours. The hydrogen is very pure, and only the simplest apparatus is required for its production. Since the powder is quite stable if protected from moisture, it should form a useful source of gas; about 1,300 litres may be obtained from one kg. of the preparation.

Hydrogen is also obtained in large quantities as a by-product in some chemical processes; for instance, in Germany the Badische Anilin und Soda Fabrik, the Chemische Fabrik Griesheim, and the Deutschen Solvay Werke produce annually twenty to twenty-five million c.m. of chemically pure hydrogen, which, being of no use to them, is simply let out to escape in the air. Ample opportunity is therefore given to find means to collect and utilise such an enormous quantity of a technically useful gas.