Properties. - Oxygen is a colourless gas without smell, slightly heavier than common air. It forms rather more than a fifth by volume of the atmosphere.

Preparation. - By heating chlorate of potassium with peroxide of manganese 2KClO3 = 2KCl + 3O2

Peroxide of manganese merely aids the decomposition of the chlorate of potassium, and takes no part in the reaction.

Physiological Action. - Oxygen applied to the unbroken skin has but little action, but when applied to a wound it increases the circulation in it, and acts as a stimulant. When inhaled by healthy persons it causes a slight feeling of warmth in the mouth, extending downwards over the front of the body. In some people it appears to cause nervous symptoms somewhat like those produced by nitrous oxide.

In animals, excess of oxygen produces tetanic symptoms almost like those of strychnine, and death. This effect is produced by a pressure of three atmospheres, and it is evident that it is due to the oxygen and not to the simple increase in atmospheric pressure only, because when ordinary air is used, a pressure of three atmospheres has no such action, and a pressure of twenty-five atmospheres is requisite to produce this effect (Bert).

It has been thought by some that when oxygen has been once breathed it loses something which enables it to support life. The reason of this belief is that animals soon die which are kept in a confined space, from which the carbonic acid formed during respiration is absorbed by lime or baryta, and its place supplied by fresh oxygen. Professor Seegen, however, has found that the death in such cases is not due to the removal of anything from the oxygen, but to actual poisoning by the products of tissue-waste. In some experiments he noticed that the air in which the animal had been confined for a while, although its chemical composition was correct, had a disagreeable smell, and the animal after its removal soon died of pneumonia. When the air which the animal was breathing was extracted from one end of the compartment, made to pass through a red-hot tube, and introduced at the other end so that any organic matter formed during respiration was consumed, the animal could be kept for almost any length of time without injury to its health.

Uses. - Oxygen has been applied to the surface in atonic, scrofulous, and syphilitic ulcers, and in cases of senile or other gangrene. It has more especially been employed in cases of respiratory disease, such as emphysema, bronchial dilatation, phthisis, and gangrene of the lung, in asphyxia from noxious vapours or anaesthetics, and in spasmodic asthma. It seems to be chiefly of use in the latter disease. It has been employed also in cases of difficulty of breathing, of cardiac disease, and of anaemia from loss of blood or suppuration. It has been employed also in conditions where oxidation seems to be deficient, as in gout and diabetes, where sometimes the sugar disappears from the urine during its inhalation. Oxygen has also been used in the treatment of epilepsy and spasm.

It has been strongly recommended by Bert in paralysis occurring in divers, due to their sudden ascension from a great depth to the surface. When submerged at a considerable depth the pressure of the air causes both nitrogen and oxygen to be absorbed by the blood; when they return to the surface the oxygen enters into combination, but the nitrogen is set free in the blood-vessels, forming minute bubbles, which act as emboli, obstructing the circulation in the nerve-centres and in the lungs, thus producing paralysis and dyspnoea. The nitrogen diffuses as readily into an atmosphere of oxygen as into an absolute vacuum; and therefore when animals, in which such a state has been artificially induced, have been made to breathe pure oxygen, bubbles of nitrogen disappear from the blood, the circulation is speedily restored to its normal condition, and the paralysis and dyspnoea disappear.

Its inhalation has been recommended in cases of cholera.