Oxygen

Carl Wilhelm Scheele, working as an apothecary in Sweden, prepared oxygen at some point in 1771 or 1772, described it as fire air, and did not publish until 1777. Joseph Priestley, in England, obtained it on 1 August 1774 by focusing sunlight onto mercuric oxide, noted that a candle burned in it with remarkable vigour and that a mouse lived unusually long in it, and published in 1775. Antoine Lavoisier, in Paris, heard of Priestley's result from Priestley himself over dinner in October 1774, repeated the experiment, and understood it.

That is the whole dispute, and every part of it is true. Scheele made it first. Priestley published first. Lavoisier explained it, named it, and built a theory of chemistry on it. National accounts have tended to promote whichever of the three suits them, and the honest summary is that the discovery was genuinely divided and that the three contributions were of different kinds.

What Priestley and Scheele had was a gas. What Lavoisier had was combustion. The prevailing account held that burning bodies release a substance called phlogiston; Lavoisier showed that they instead combine with something drawn from the air, and that the products weigh more than the starting materials rather than less. He named the gas oxygene, acid-former, on the mistaken belief that it is present in all acids, and the error is fossilised in the name.

The consequence reaches beyond chemistry. Phlogiston was the last framework in which alchemical practice and academic chemistry still shared a vocabulary. Lavoisier's list of elements in 1789, and his insistence that an element is simply whatever cannot be broken down further by experiment, replaced a system of principles and qualities with a system of substances. Alchemy did not so much lose the argument as lose the language the argument had been conducted in.

Oxygen is the most abundant element in the Earth's crust, about 46 per cent by mass, but nearly all of it is locked into silicate rock, where it is doing nothing. Free oxygen in the atmosphere is a different matter, and it is an anomaly.

Oxygen is reactive. Left alone it combines with iron, with carbon, with sulfur, with almost anything, and within a geologically short time it would be gone. That the air is a fifth oxygen means something is producing it continuously. That something is photosynthesis.

For roughly the first half of Earth's history there was effectively none. Cyanobacteria began splitting water for its hydrogen and discarding the oxygen, and from about 2.4 billion years ago the free gas began to accumulate, in what is called the Great Oxidation Event. Banded iron formations, the layered red rock that supplies most of the world's iron ore, are the record of dissolved iron in the ancient oceans being precipitated out by that new oxygen. For much of the life then existing, the change was a catastrophe.

Aerobic respiration, which extracts far more energy from food than any anaerobic route, became possible only afterwards, and every complex organism depends on it. Ozone, a three-atom form of oxygen concentrated in the stratosphere, absorbs the ultraviolet that would otherwise make the land surface uninhabitable, and it too is a product of the free oxygen beneath it.

Liquid oxygen is pale blue and magnetic, clinging to the poles of a magnet, which is a directly visible consequence of the two unpaired electrons in the ordinary oxygen molecule. Medical use of concentrated oxygen dates from the late nineteenth century; industrial use, chiefly in steelmaking, consumes far more of it.