(O. Eq.=8.)

Oxygen is a colourless and odourless gas; it forms, on an average, 20.81 per cent. by measure, or 23.01 per cent. by weight, of atmospheric air. The specific gravity is 1.1057, the specific gravity of air being 1. It combines in various proportions with nearly all the elementary substances, and is the great supporter of combustion and respiration. In combination with hydrogen it forms water.

Oxygen may be prepared in various ways; when large quantities are required it is generally obtained by the action of heat on peroxide of manganese, 3 MnO2=MnO, Mn2O3 + 2 0, pure oxide of manganese yielding about one-ninth of its weight of oxygen. Chlorate of potash may be used for the same purpose, and the decomposition is then as follows: K0,C105=Kc1 + 6 0.

A peculiar modification of oxygen, known as Ozone, is produced under certain circumstances, as when a succession of electric sparks is passed through air, or through oxygen, a large portion of the oxygen is converted into ozone, which has a peculiar odour, is much denser than oxygen, and seems to be a more active form of this gas; it can be formed by other means, and in the course of the slow oxidation of phosphorous and other substances, is produced in small quantities.

It is a powerful oxidising agent; it displaces iodine from its metallic combinations; starch paper impregnated with a solution of iodide of potassium is generally used to detect its presence in the atmosphere, the iodine being set free combines with the starch to form the blue iodide; it converts the protosalts of manganese into persalts, and the sulphite of lead into sulphate.

Therapeutics. A solution of oxygen in water has been used as a slight stimulant and excitant. Considerable advantages were at first anticipated from the introduction of the gas as a remedial agent, but clinical experience has not confirmed these anticipations. Its inhalation, which has been tried in certain states of the system accompanied with deficient aeration of the blood, has not been attended with much success.