Oxygen was discovered almost simultaneously in the year 1774 by Priestley and by Scheele, the Swedish chemist having, however, nearly completed his discovery in 1772.

Priestley discovered that the red oxide of mercury evolved a gas when heated. This gas, oxygen, being superior even to the air as a supporter of combustion, was regarded by him as "dephlogisticated air." The incombustible part of the atmosphere he supposed to be saturated with phlogiston on the assumption that a gas was so much the better adapted for supporting combustion, as it contained within itself a smaller quantity of that body, common air, by drawing phlogiston from burning substances, because, as he thought, phlogisti-cated air on that account had no longer any attractions for phlogiston, or, in other words, any power of supporting combustion.

In 1789 Lavoisier, who by a series of carefully conducted and very ingenious experiments proved that the combustion of bodies in the air consisted essentially in their chemical combination with oxygen, and thus overthrew the "phlogiston " theory, gave it the name which it now retains (from oxys = acid and gennao = I produce), in consequence of his (erroneously) believing that it was a necessary constituent of every acid.

Oxygen was liquefied in 1877 by Pictet at a pressure of 320 atmospheres and a temperature of - 140°. Wroblewski and Olszewski have shown that the critical temperature of oxygen (i.e., the temperature above which no amount of pressure will liquefy it) is - 113°, the pressure needed to liquefy it at that temperature being 50 atmospheres. Its boiling point is - 181.4 at ordinary pressure. When the pressure is reduced or removed, evaporation takes place so rapidly that a part of the oxygen is often frozen to a white solid. Under 13.7 atmospheres solidification takes place at - 146.8. Sir James Dewar is endeavouring to obtain' liquid oxygen at atmospheric pressure, and in 1892 he devised a vacuum vessel for containing liquid oxygen.

In 1808 Gay-Lussae made known to the world the laws of the combination of gases by volume, to which his attention had been directed by the discovery which he and Alex. v. Humboldt had made that a definite volume of oxygen combined with exactly twice its bulk of hydrogen. He pointed out that there is a simple relation between the volumes of two gases which unite together and also between their collective volume in the uncombined and in the combined condition.

According to the law of Boyle and Mariotte the volume of a given mass of any gas varies inversely as the pressure, provided that the temperature remains the same; for instance, the quantity of air which is contained in a vessel of the capacity of one pint under the pressure of one atmosphere, or 15 lbs. upon the square inch, may be contained in a vessel of half a pint capacity if the pressure be doubled.

According to the law of Charles and Gay-Lussac, on the other hand, all gases expand equally by heat, provided the pressure remains constant, the rate of expansion being 1/273 of the volume at 0° C. for each rise of 1° C. in temperature ; or, in other words, the volume of a gas varies directly as the absolute temperature. A gas which strictly conforms to these two laws is said to be a perfect gas, but none of the gases with which we are acquainted are perfect in this sense.

From the few accurate observations which have been made on this subject it appears that, in general, the departure from the laws of Boyle and Charles is greater the more the temperature of the gas approaches to that at which it becomes liquid. The general resemblance in the behaviour of gases under the influence of pressure and heat is very great, however.

Numerous processes have been devised for the industrial production of oxygen, but most of them are so expensive, or require such complicated plants, that only two or three are in actual operation on a large scale.

Erin's process of producing oxygen by the alternate formation and decomposition of barium peroxide is an improvement upon the Bausingault process of 1851 and is being worked by The British Oxygen Company, Limited. The installation of the plant requires a considerable space, and special heating arrangements are required underground so as to produce a working temperature of some 800° Fahr. The process must be worked day and night, as, according to the nature of the process, the oxidation takes place every five minutes - that is to say, no oxygen is being produced; besides, the furnace cannot be left to cool during the night, as the success of the process depends upon certain fixed temperatures. The smallest change in temperature in the furnace, the smallest pollution of the air, when the purifier does not act properly, and the baryte itself being apt to change entirely in the furnace, may produce losses of considerable extent during a few days' time.

Brin's process has therefore many disadvantages, and its success depends upon so many contingencies that it cannot favourably compare with other processes which may be considered safe from a technical point of view.

The Production of Oxygen by Electrolysis of Water has of late found considerable extension, although it was considered that the inevitable loss attending the conversion of heat into power and power into electrical force, and the need of skilled labour, would make the process too expensive.

Amongst the various patents five different processes have found practical application, viz., those of Garuti, Schuckert, Dr. Schmidt (Zurich), Hasard Flamand, and Renard.

All these processes are based upon the decomposition of alkaline solutions by means of the electric current liberating two volumes of hydrogen and one volume of oxygen in accordance with the formula H2O = 2 HO. They differ, however, in the type of electrodes employed. Schmidt and Renard work with porous diaphragms of non-conducting material, while in the processes of Garuti and Schuckert perforated partitions of a conducting material are used.