~The ammonium radical.~ The radical NH_{4} plays the part of a metal in many chemical reactions and is called ammonium. The ending _-ium_ is given to the name to indicate the metallic properties of the substance, since the names of the metals in general have that ending. The salts formed by the action of the base ammonium hydroxide on acids are called ammonium salts. Thus, with hydrochloric acid, ammonium chloride is formed in accordance with the equation NH_{4}OH + HCl = NH_{4}Cl + H_{2}O. Similarly, with nitric acid, ammonium nitrate (NH_{4}NO_{3}) is formed, and with sulphuric acid, ammonium sulphate ((NH_{4})_{2}S0_{4}). It will be noticed that in the neutralization of ammonium hydroxide by acids the group NH_{4} replaces one hydrogen atom of the acid, just as sodium does. The group therefore acts as a univalent metal. ~Combination of nitrogen with hydrogen by volume.~ Under suitable conditions ammonia can be decomposed into nitrogen and hydrogen by passing electric sparks through the gas. Accurate measurement has shown that when ammonia is decomposed, two volumes of the gas yield one volume of nitrogen and three volumes of hydrogen. Consequently, if the two elements were to combine directly, one volume of nitrogen would combine with three volumes of hydrogen to form two volumes of ammonia. Here, as in the formation of steam from hydrogen and oxygen, small whole numbers serve to indicate the relation between the volumes of combining gases and that of the gaseous product. COMPOUNDS OF NITROGEN WITH OXYGEN AND HYDROGEN In addition to ammonium hydroxide, nitrogen forms several compounds with hydrogen and oxygen, of which nitric acid (HNO_{3}) and nitrous acid (HNO_{2}) are the most familiar. ~Nitric acid~ (HNO_{3}). Nitric acid is not found to any extent in nature, but some of its salts, especially sodium nitrate (NaNO_{3}) and potassium nitrate (KNO_{3}) are found in large quantities. From these salts nitric acid can be obtained. [Illustration Fig. 37] ~Preparation of nitric acid.~ When sodium nitrate is treated with concentrated cold sulphuric acid, no chemical action seems to take place. If, however, the mixture is heated in a retort, nitric acid is given off as a vapor and may be easily condensed to a liquid by passing the vapor into a tube surrounded by cold water, as shown in Fig. 37. An examination of the liquid left in the retort shows that it contains sodium acid sulphate (NaHSO_{4}), so that the reaction may be represented by the equation NaNO_{3} + H_{2}SO_{4} = NaHSO_{4} + HNO_{3}. If a smaller quantity of sulphuric acid is taken and the mixture is heated to a high temperature, normal sodium sulphate is formed: 2NaNO_{3} + H_{2}SO_{4} = Na_{2}SO_{4} + 2HNO_{3}. In this case, however, the higher temperature required decomposes a part of the nitric acid.