Gold

Gold is the Sun. The symbol is a circle with a dot at its centre, which is also the astronomical sign for the Sun and one of the oldest ideograms still in daily use. In alchemical Latin the metal is routinely called Sol, and preparations of it are solar.

Unlike tin and Jupiter, this correspondence needs no reconstruction. Gold is yellow, it is bright, it does not tarnish, and it is the most valued of the metals; the Sun is yellow, bright, unchanging and the chief of the heavenly bodies. The reasoning sits on the surface of the thing, and the association appears in Egyptian, Mesopotamian, Greek, Indian and Chinese contexts independently of one another.

The physical basis of the colour is worth stating, because it is unusual. Almost all metals are silvery-white; they reflect visible light more or less evenly. Gold absorbs in the blue and reflects yellow and red. The reason is relativistic: gold's inner electrons move fast enough that relativistic effects contract the orbitals and reduce the energy gap between two particular electron levels, bringing the absorption down out of the ultraviolet, where it sits in silver, and into the visible blue. Gold is yellow because of special relativity, which is a fact worth having, and cesium and copper acquire their colour by related routes.

Gold does not corrode. It does not react with oxygen at any temperature, it is unaffected by water, and it resists every single acid. It dissolves only in aqua regia, royal water, one part nitric acid to three of hydrochloric, a preparation described in Arabic alchemical texts by the thirteenth century and known throughout the Latin tradition afterwards. The name says exactly what it means: the water that dissolves the king of metals. Gold's permanence is why grave goods come out of the ground still bright, and it is the physical fact standing behind every association of gold with incorruptibility and with eternity.

Transmutation of base metals into gold was the central practical aim of a great deal of alchemy, and it had a name, chrysopoeia, gold-making. It is worth being clear about why intelligent people pursued it for fifteen hundred years, because the usual dismissal misses the point.

Within the sulfur-mercury theory, metals were compounds of two principles in varying proportion and purity. Gold was the perfectly balanced case, lead the corrupted one. On that theory transmutation is not magic; it is refinement, and it is exactly as reasonable an undertaking as purifying an ore. The theory was wrong, but it was a coherent chemical theory, and the project followed from it by ordinary logic.

Laboratory results reinforced it. Adding arsenic or zinc to copper produces alloys that look like silver and like gold. Gilding and amalgamation produce gold-coloured surfaces. Dissolving gold in aqua regia makes it vanish; precipitating it brings it back. To an observer without an atomic theory these are transmutations, and the difference between changing the appearance of a metal and changing the metal itself is not obvious.

The work was not wasted. Distillation apparatus, the mineral acids, laboratory glassware, the systematic recording of procedure, and the isolation of phosphorus and of a range of antimony and arsenic compounds all came out of it. Robert Boyle and Isaac Newton both worked seriously on transmutation, and Newton's alchemical manuscripts are extensive.

Transmutation is in fact possible, and it has been done. In 1980 Glenn Seaborg's group at Berkeley converted a minute quantity of bismuth into gold by stripping protons from the nuclei in a particle accelerator. The result cost immeasurably more than the gold was worth, which is the honest ending to the story: the goal was real, the method was wrong, and the method that works is uneconomic.

Gold is rare for a reason that has nothing to do with the Earth. Elements heavier than iron cannot be made by ordinary stellar fusion, because fusing them consumes energy rather than releasing it. They are built by neutron capture in rare and violent events: supernovae, and above all the collision of neutron stars. Observation of the neutron star merger detected by its gravitational waves in August 2017 showed the spectroscopic signature of heavy element production directly. Gold is rare because the events that make it are rare.

It is rarer still at the surface than in the planet as a whole, because gold is dense and dissolves readily in molten iron, so most of the Earth's gold sank into the core during formation. The gold that can be mined is generally thought to have arrived later, in the bombardment that followed.

Crustal abundance is around four parts per billion. The total quantity ever mined is roughly 200,000 tonnes, which would form a cube about twenty-two metres on a side. Most of it still exists, because gold is not consumed.

Its uses divide sharply. Jewellery and investment take the great majority. Industrially, gold's value is that it does not corrode and conducts well, which makes it the standard plating for electrical contacts and connectors wherever reliability matters absolutely; there is gold in every phone, every computer and every spacecraft. Gold leaf, beaten to about a ten-thousandth of a millimetre, coats domes and manuscripts and the visors of spacesuits, where it reflects infrared. Colloidal gold, particles a few nanometres across, is red rather than yellow, was used to colour medieval glass, and is now a standard reagent in medical testing: the coloured line on a lateral flow test is gold particles.