~General.~ As explained in a previous chapter, when ammonia is passed into water the two compounds combine to form the base NH_{4}OH, known as ammonium hydroxide. When this base is neutralized with acids there are formed the corresponding salts, known as the ammonium salts. Since the ammonium group is univalent, ammonium salts resemble those of the alkali metals in formulas; they also resemble the latter salts very much in their chemical properties, and may be conveniently described in connection with them. Among the ammonium salts the chloride, sulphate, carbonate, and sulphide are the most familiar. ~Ammonium chloride~ (_sal ammoniac_) (NH_{4}Cl). This substance is obtained by neutralizing ammonium hydroxide with hydrochloric acid. It is a colorless substance crystallizing in fine needles, and, like most ammonium salts, is very soluble in water. When placed in a tube and heated strongly it decomposes into hydrochloric acid and ammonia. When these gases reach a cooler portion of the tube they at once recombine, and the resulting ammonium chloride is deposited on the sides of the tube. In this way the salt can be separated from nonvolatile impurities. Ammonium chloride is sometimes used in preparation of ammonia; it is also used in making dry batteries and in the laboratory as a chemical reagent. ~Ammonium sulphate~ ((NH_{4})_{2}SO_{4}). This salt resembles the chloride very closely, and, being cheaper, is used in place of it when possible. It is used in large quantity as a fertilizer, the nitrogen which it contains being a very valuable food for plants. ~Ammonium carbonate~ ((NH_{4})_{2}CO_{3}). This salt, as well as the acid carbonate (NH_{4}HCO_{3}), is used as a chemical reagent. They are colorless solids, freely soluble in water. The normal carbonate is made by heating ammonium chloride with powdered limestone (calcium carbonate), the ammonium carbonate being obtained as a sublimate in compact hard masses: 2NH_{4}Cl + CaCO_{3} = (NH_{4})_{2}CO_{3} + CaCl_{2}. The salt always smells of ammonia, since it slowly decomposes, as shown in the equation (NH_{4})_{2}CO_{3} = NH_{4}HCO_{3} + NH_{3}. The acid carbonate, or bicarbonate, is prepared by saturating a solution of ammonium hydroxide with carbon dioxide: NH_{4}OH + CO_{2} = NH_{4}HCO_{3}. It is a well-crystallized stable substance. ~Ammonium sulphide~ ((NH_{4})_{2}S). Ammonium sulphide is prepared by the action of hydrosulphuric acid upon ammonium hydroxide: 2NH_{4}OH + H_{2}S = (NH_{4})_{2}S + 2H_{2}O. If the action is allowed to continue until no more hydrosulphuric acid is absorbed, the product is the acid sulphide, sometimes called the hydrosulphide: NH_{4}OH + H_{2}S = NH_{4}HS + H_{2}O. If equal amounts of ammonium hydroxide and ammonium acid sulphide are brought together, the normal sulphide is formed: NH_{4}OH + NH_{4}HS = (NH_{4})_{2}S + H_{2}O