Ozone is a modification, or allotropie form of oxygen, which was discovered in 1840. It is composed solely of oxygen, although its properties are markedly different. The difference is explained by the assumption, that in ozone three atoms of oxygen are united to form a single molecule, whereas in oxygen itself the molecule consists of but two atoms. That three volumes of oxygen form two volumes of ozone was shown by the experiments of Andrews and of Sorel. It is formed during a silent electric discharge through oxygen, and is produced in small quantities during electrolysis, and most slow oxidations, e.g. of phosphorus. It possesses a powerful penetrating odour, and acts as a more powerful oxidiser than oxygen, acting on many substances unaffected by the latter element. At low temperatures it may be condensed to a blue liquid. It is present to a slight extent in the atmosphere near the sea and in the open country, but in the neighbourhood of towns it is absent, being used up in destroying the organic material also present in the air.