Nitric oxide, also called nitrogen monoxide, is a colourless toxic gas that is formed by the oxidation of nitrogen. Nitric oxide is formed from nitrogen and oxygen by the action of electric sparks or high temperatures or, more conveniently, by the action of dilute nitric acid upon copper or mercury. It was first prepared about 1620 by the Belgian scientist Jan Baptista van Helmont, and it was first studied in 1772 by the English chemist Joseph Priestley, who called it nitrous air. The gas is almost insoluble in water, but it dissolves rapidly in a slightly alkaline solution of sodium sulfite, forming the compound sodium dinitrososulfite. It reacts rapidly with oxygen to form nitrogen dioxide. Nitric oxide is a relatively unstable, diatomic molecule that possesses a free radical. In the chemical industry, nitric oxide is an intermediate compound formed during the oxidation of ammonia to nitric acid. An industrial procedure for the manufacture of hydroxylamine is based on the reaction of nitric oxide with hydrogen in the presence of a catalyst. The formation of nitric oxide from nitric acid and mercury is applied in a volumetric method of analysis for nitric acid or its salts. Nitric oxide is an important component of the air pollution generated by automotive engines and thermal power-generating plants. When a mixture of air and hydrocarbon fuel is burned in an internal-combustion engine or a power plant, the ordinarily inert nitrogen in the air combines with oxygen at very hig