Mercury

Mercury is the one metal that is liquid at ordinary temperature. It freezes only at minus 38.8 degrees C, and it flows, beads and shatters into rolling globules that recombine on contact. To anyone without a modern theory of matter this is a metal behaving as a liquid, and it was entirely reasonable to conclude that it was the metallic principle itself in its most fluid state.

That is precisely the role it plays in alchemical theory. In the sulfur-mercury account, current in Latin Europe from the twelfth century and inherited from Arabic sources, every metal is composed of a mercurial principle giving fusibility, lustre and volatility, and a sulfurous principle giving combustibility and fixity. Ordinary quicksilver was the visible embodiment of the first, and the texts distinguish it, sometimes carefully and sometimes not at all, from the philosophical mercury they are actually discussing. Paracelsus made it one of the three principles, the tria prima, in the sixteenth century.

Its planetary correspondence is Mercury, and the symbol combines a crescent above a circle above a cross: horns, disc and cross, one of the more elaborate of the seven. The link to the planet is usually explained by speed, Mercury being the fastest-moving of the planets and quicksilver the most mobile of the metals, and by the god's character as messenger, trickster and crosser of boundaries, which suits a substance that is neither properly solid nor properly liquid. The Latin name of the metal, hydrargyrum, water-silver, gives the symbol Hg; quicksilver, from the Old English for living silver, says the same thing.

Mercury's principal ore is cinnabar, mercury sulfide, and cinnabar is a spectacular material in its own right: a deep saturated scarlet, ground since the neolithic as a pigment and as a burial powder. Ground and purified it is vermilion, the most brilliant red available to painters before modern synthetics, used from Roman wall painting through Chinese lacquer to European panel and manuscript work. Chinese vermilion was being made synthetically from mercury and sulfur by at least the Tang period.

The mines at Almaden in Spain have been worked for mercury for over two thousand years and supplied a large share of world production well into modern times.

What made cinnabar central to alchemy was not the colour but a demonstration. Heat cinnabar in air and it gives up liquid metal directly; combine that liquid metal with sulfur and it reconstitutes as a red solid. A brilliant red mineral becomes a running silver liquid and can be returned to red again, reversibly, in a simple apparatus. There is no better visible argument that substances are composed of principles which can be separated and recombined, and it was performed and re-performed for centuries as exactly that.

Chinese alchemy went further and treated cinnabar as the central substance of the elixir of immortality. Mercury and cinnabar preparations were taken internally in pursuit of long life, and several Chinese emperors are recorded as having died of them, including, on the standard account, the first emperor Qin Shi Huang. The tomb of Qin Shi Huang is described by Sima Qian as containing rivers and seas of flowing mercury, and soil surveys of the unexcavated mound have found anomalously high mercury concentrations, which is a rare case of a classical text being partially corroborated by instrument.

Mercury is a cumulative neurotoxin, and the historical record of its use is a record of harm that took a very long time to be admitted.

The vapour is the immediate danger, since liquid mercury evaporates slowly at room temperature and the vapour is absorbed efficiently by the lungs. Chronic exposure produces tremor, memory loss, insomnia, irritability and personality change. Mercuric nitrate was used to treat felt in hat-making, and the resulting syndrome among hatters gave the phrase mad as a hatter its literal origin. Mercury was mined at Almaden and at Huancavelica with convict and forced labour, and the mercury amalgamation process used to extract silver at Potosi killed on a scale that is difficult to establish and was certainly very large.

Mercury was also medicine. It was the standard treatment for syphilis from the late fifteenth century until Salvarsan in 1910, applied as an ointment, taken as pills and inhaled as fumigation, and the saying that a night with Venus leads to a lifetime with Mercury records the fact accurately. Calomel, mercurous chloride, was a general-purpose purgative given freely to adults and children into the twentieth century. Teething powders containing calomel caused a childhood syndrome called pink disease, which was not connected to mercury until the 1950s.

Organic mercury is worse than the metal. Methylmercury, formed by bacteria from inorganic mercury in water, accumulates up the food chain and crosses the placenta. Industrial discharge of mercury into Minamata Bay in Japan from 1932 caused severe neurological damage and death across the local population over decades, and gave the condition its name. Consumption advice on large predatory fish derives from the same mechanism.

The uses have been progressively withdrawn: mercury thermometers, barometers, dental amalgam, batteries, switches and fungicides. The Minamata Convention, adopted in 2013, controls mercury internationally. Artisanal gold mining, which still uses mercury amalgamation, is now the largest single source of releases.