OXYGEN (0), an elementary body, the properties of which are best known and most remarkable in its elastic or tti;riforrn state, in which it is termed oxygen gas. The most important discovery of this substance was made by Dr. Priestley, on the 1st of August, 1774; and of his numerous contributions to chemical science, this is at once the most splendid and the most solid, and has rendered his name imperishable.
Dr. Priestley has stated, with his usual candour, that the discovery was also made, quite independently, though later, by Scheele, and he has refuted the unjust claim of Lavoisier to be also one of the dis coverers of this element. To this gas Dr. Priestley gave the name of dephlogistieated air, Scheele called it empyreal air, and Condorcet vital air. Lavoisier gave it the name of oxygen, from the Greek words /5/Jr, acid, and the root re, to generate, because he considered it as the sole cause of acidity; this appellation it still retains, although it is now well known that • there are acids which do not contain it and alkalies which do.
Dr. Priestley first prepared this gaseous body by heating the binoxide of mercury, then called mercuries prcrcipitatus per se, in an air-jar, over mercury, by means of a lens, and lie afterwards obtained it from other substances.
It is now prepared in various ways, according to the purposes to which it is intended to be applied. These ways we shall briefly state and explain, and also describe the production of the gas from the bin oxide of mercury, first used by Dr. Priestley.
When mercury is heated, and st the same time exposed to atmos pheric air, it is found that the volume of the air is diminished, and the weight of the mercury increased, and that it becomes, during the operation, is red crystalline body, which is the binoxide of mercury, formed by the metal combining with the oxygen of the air.
NVhen this substance is subjected in a retort to a higher temperature than that required for its formation, the affinity between the metal and the oxygen is overcome ; the former returns to its metallic state, and the latter to its gaseous form ; and if added to the residual atmospheric air, its volume is restored.
If it be quite pure, and the operation be accurately performed, 216 grains (1 equivalent) of binoxlde of mercury should be separated by the agency of heat into 200 grains, one equivalent of mercury, and 16 grains, two equivalents of oxygen, measuring 46.7 cubic inches.
By this process then oxygen gas is, though indirectly, obtained from atmospheric air : but there is a natural metallic oxide, the binoxide of manganese, which is found plentifully, and which also yields it by the mere application of beat, and the quantity of gas obtained is dependent upon the degree of heat employed; in this case, however, unlike that of the binoxide of mercury, the metal is not reduced. The equivalent of binoxide of manganese [Masmasese] is 43.57, consisting of of metal and 2 equivalents of oxygen =16 ; and when grains are heated to moderate redness in a coated glass retort, half an equivalent of oxygen, or 4 grains, measuring 11.67 cubic inches, are expelled, and there remains in the retort an equivalent of seaquioxide of manganese, composed of metal and 12 oxygen But if the equivalent of binoxide be strongly heated in an iron retort, it is then converted into red oxide, consisting of one equivalent of metal, and one and one-third of oxygen, while two-thirds of an equivalent of oxygen aro expelled, weighing 513 grains and measuring cubic inches.
Another but leas eligible method of obtaining oxygen from the bin oxide of manganese, is to heat it with sulphuric acid in a retort ; in this case an equivalent of the binoxide= loses a whole equivalent of oxygen= 8 grains, or cubic inches, and there remain in the retort protoxide of manganese combined with the sulphuric acid, forming protosulphate of manganese. It is to be observed that these statements of the quantities of oxygen procurable from a given quantity of the binoxide of manganese are made on the supposition of its being quite pure, which is hardly ever the case; the binoxido of commerce very commonly contains 20 per cent. of impurity.