Hydrogen, besides being contained in water, is also contained in many other substances,[18] and may be obtained from them. As examples of this, it may be mentioned (1) that a mixture of formate of sodium, CHNaO_{2}, and caustic soda, NaHO, when heated to redness, forms sodium carbonate, Na_{2}CO_{3}, and hydrogen, H_{2};[19] (2) that a number of organic substances are decomposed at a red heat, forming hydrogen, among other gases, and thus it is that hydrogen is contained in ordinary coal gas. [18] Of the metals, only a very few combine with hydrogen (for example, sodium), and give substances which are easily decomposed. Of the non-metals, the halogens (fluorine, chlorine, bromine, and iodine) most easily form hydrogen compounds; of these the hydrogen compound of chlorine, and still more that of fluorine, is stable, whilst those of bromine and iodine are easily decomposed, especially the latter. The other non-metals--for instance, sulphur, carbon, and phosphorus--give hydrogen compounds of different composition and properties, but they are all less stable than water. The number of the carbon compounds of hydrogen is enormous, but there are very few among them which are not decomposed, with separation of the carbon and hydrogen, at a red heat. [19] The reaction expressed by the equation CNaHO_{2} + NaHO = CNa_{2}O_{3} + H_{2} may be effected in a glass vessel, like the decomposition of copper carbonate or mercury oxide (_see_ Introduction); it is non-reversible, and takes place without the presence of water, and therefore Pictet (_see_ later) made use of it to obtain hydrogen under great pressure. Charcoal itself liberates hydrogen from steam at a high temperature;[20] but the reaction which here takes place is distinguished by a certain complexity, and will therefore be considered later. [20] The reaction between charcoal and superheated steam is a double one--that is, there may be formed either carbonic oxide, CO (according to the equation H_{2}O + C = H_{2} + CO), or carbonic anhydride CO_{2} (according to the equation 2H_{2}O + C = 2H_{2} + CO_{2}), and the resulting mixture is called _water-gas_; we shall speak of it in Chapter IX.