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Oxygen : Oxygen, Occurrence



The element oxygen occurs in the free state as a gas, of the molecular formula O2. It is colourless, odourless, and tasteless, and supports combustion and respiration. Oxygen occurs to the extent of 21 per cent, by volume or 23 per cent, by weight in the atmosphere, and takes part in processes of combustion; its biological functions in respiration make it important. The gas is sparingly soluble in (and may therefore be collected over) water, but the small quantity of oxygen dissolved in river and sea waters is essential to the life offish. Its solubility in sea water is about 0.78 that in pure water.

Combined oxygen occurs in water, in vegetable and animal tissues, in nearly all rocks and in many minerals: it occurs to a larger extent (about 50 per cent.) in the earth's crust than any other element.

Oxygen was first isolated by Scheele in 1772, and was discovered independently by Priestley in 1774.

The Egyptian chemist Zosimos (250 a.d.) refers to a gaseous body evolved on heating a substance floating on the surface of heated mercury (possibly mercury oxide). According to Klaproth, the Chinese philosopher Mao Khoa (750 a.d.) assumed two primary elements: Yin (the weak), and Yang (the strong). In air, yin and yang are combined with fire. When charcoal is burnt in air, yang is left, whilst yin could be obtained by heating a substance Hhò-siaô (possibly nitre).




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