The yellow modification is prepared by adding a solution of a mercuric salt to a solution of sodium or potassium hydroxide: Hg(NO_{3})_{2} + 2NaOH = 2NaNO_{3} + Hg(OH)_{2}, Hg(OH)_{2} = HgO + H_{2}O. When heated the oxide darkens until it becomes almost black; at a higher temperature it decomposes into mercury and oxygen. It was by this reaction that oxygen was discovered. ~Mercurous chloride~ (_calomel_) (HgCl). Being insoluble, mercurous chloride is precipitated as a white solid when a soluble chloride is added to a solution of mercurous nitrate: HgNO_{3} + NaCl = HgCl + NaNO_{3}. Commercially it is manufactured by heating a mixture of mercuric chloride and mercury. When exposed to the light it slowly changes into mercuric chloride and mercury: 2HgCl = HgCl_{2} + Hg. It is therefore protected from the light by the use of colored bottles. It is used in medicine. Most mercurous salts are insoluble in water, the principal soluble one being the nitrate, which is made by the action of cold, dilute nitric acid on mercury. ~Mercuric chloride~ (_corrosive sublimate_) (HgCl_{2}). This substance can be made by dissolving mercuric oxide in hydrochloric acid. On a commercial scale it is made by subliming a mixture of common salt and mercuric sulphate: 2NaCl + HgSO_{4} = HgCl_{2} + Na_{2}SO_{4}. The mercuric chloride, being readily volatile, vaporizes and is condensed again in cool vessels. Like mercurous chloride it is a white solid, but differs from it in that it is soluble in water. It is extremely poisonous and in dilute solutions is used as an antiseptic in dressing wounds. ~Mercuric sulphide~ (HgS). As cinnabar this substance forms the chief native compound of mercury, occurring in red crystalline masses. By passing hydrosulphuric acid into a solution of a mercuric salt it is precipitated as a black powder, insoluble in water and acids. By other means it can be prepared as a brilliant red powder known as vermilion, which is used as a pigment in fine paints. ~The iodides of mercury.~ If a solution of potassium iodide is added to solutions of a mercurous and a mercuric salt respectively, the corresponding iodides are precipitated. Mercuric iodide is the more important of the two, and as prepared above is a red powder which changes to yellow on heating to 150 deg.. The yellow form on cooling changes back again to the red form, or may be made to do so by rubbing it with a knife blade or some other hard object. SILVER ~Occurrence.~ Silver is found in small quantities in the uncombined state; usually, however, it occurs in combination with sulphur, either as the sulphide (Ag_{2}S) or as a small constituent of other sulphides, especially those of lead and copper. It is also found alloyed with gold.