Oxygen (sym. O, atom. wt. 16) is a colourless, odourless, tasteless gas, long regarded as a 'permanent' gas, but liquefied by Pictet of Geneva for the first time in 1877. Its chemical affinities for other elementary substances are very powerful; with most of them it is found in combination, or may be made to combine, in more than one proportion; with several in as many as four different proportions; and there is only one element (fluorine) with which it does not enter into any combination.
Owing to the intensity with which many of these combinations take place, this gas has the power of supporting Combustion (q.v.) in an eminent degree. It is only slightly soluble in water; 100 cubic inches of that liquid dissolving 4.11 cubic inches of gas at 32°, and only 2.99 inches at 59°. It is slightly heavier than air, its specific gravity being 1.1056.
Oxygen gas is not only respirable, but is essential to the support of animal life; and hence it was termed vital air by some of the older chemists. A small animal placed in a bell-glass containing pure oxygen will not be suffocated as soon as if it were placed in the same glass filled with atmospheric air. For further details on this property of oxygen, the reader is referred to the article RESPIRATION.
Oxygen is the most abundant and the most widely distributed of all the elements. In its free state (mixed but not combined with nitrogen) it constitutes about a fifth of the bulk, and considerably more than a fifth of the weight, of the atmosphere. In combination with hydrogen, it forms eight-ninths of all the water on the globe; and in combination with silicon, calcium, aluminium, &c., it enters largely into all the solid constituents of the earth's crust; silica—in its various forms of sand, common quartz, flint, &c.—chalk, limestone, marble, and all the varieties of clay, containing about half their weight of oxygen. It is, moreover, found in the tissues and fluids of all forms of animal and vegetable life, none of which can support existence independently of this element.
There are various laboratory methods of obtaining oxygen on the small scale, the simplest of which consists in the exposure of certain metallic oxides to a high temperature. It was originally obtained by its discoverer, Dr Priestley, from the red oxide of mercury, which, when heated to about 750°, resolves itself into metallic mercury and oxygen gas. It may be obtained similarly from red oxide and peroxide of lead, the resulting products being protoxide of lead and oxygen.
The ordinary laboratory method commonly employed to obtain an abundant supply of oxygen consists in heating chlorate of potash, , which yields up all its oxygen (amounting to 39.16 per cent.), and leaves a residue of chloride of potassium. One ounce of this salt yields nearly two gallons of oxygen gas. It is found by experiment that if the chlorate of potash is mixed with about a fourth of its weight of black oxide of copper, or of binoxide of manganese, the evolution of the gas is greatly facilitated, although the oxides do not seem to undergo any change during the process.
Various processes have been proposed for obtaining oxygen on the large scale, but only in recent years has the commercial production of the gas been carried out sufficiently cheaply to enable oxygen to be employed extensively for industrial purposes. The method employed by Brin's Oxygen Company consists in passing air under pressure over barium oxide, , heated to a temperature of dull redness. In this way a quantity of barium peroxide, , is formed, and this can be made to again yield up its extra oxygen in the pure state (being reduced again to ) by heating to a full red heat, or, as is actually done in practice, by greatly diminishing the gaseous pressure without altering the temperature. It is estimated that oxygen can be produced by this process at a cost of from 5s. to 7s. 6d. per 1000 cubic feet. Oxygen can now be obtained in practically any required quantity in wrought-steel cylinders, in which it is compressed up to a pressure of 120 atmospheres.
Of the compounds of oxygen it is unnecessary to speak here, as they are described in the articles on the other chemical elements.
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 called it Dephlogisticated Air; Scheele termed it Empyrean Air; Condorcet shortly afterwards suggested Vital Air, as its most appropriate designation; and 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 (q.v.) theory, gave it the name which it now retains (from oxys, 'acid,' and gennaō, 'I produce'), in consequence of his (erroneously) believing that it was a necessary constituent of every acid.