The noble metals
Back in the old days, alchemists used more evocative names for the chemicals they worked with than we do nowadays. Gold has been used as currency and ornamentation in countless cultures for thousands of years; it is soft enough to work with unsophisticated tools and — in contrast to most other metals — it does not tarnish or corrode over time. Its unique color and seemingly mystical ability to remain uncorrupted earned it particular respect from medieval experimenters. For that reason, gold and a few other metals like it were called the "noble metals".
Another evocative name was aqua regia, or "royal water", a mixture of acids with the unique property of being able to dissolve many of the noble metals. Strong acids will attack most metals, and gradually corrode or dissolve them. Dropping a piece of zinc into hydrochloric acid, for instance, will result in a stream of bubbles of hydrogen gas and given enough time, the complete dissolution of the zinc. But the noble metals not only resisted the corrupting effects of the atmosphere and water, but those of strong acids as well — except for aqua regia. Aqua regia is a mixture of concentrated hydrochloric and nitric acids, usually three to four parts of the former to one of the latter. In appearance, it is yellowish and produces heavy fumes of nitrosyl chloride (NOCl) and chlorine, which are highly volatile. As these fumes evaporate, the aqua regia loses its effectiveness, so it should be mixed immediately before use.
Dissolving a noble metal
Many chemical reactions can work in two directions; the products can undergo the same reaction in reverse to yield the original reactants again. Such reactions eventually reach an equilibrium with some quantity of the materials on each side of the equation. Even if a reaction proceeds only very slowly in one direction, some small quantity of those products will still exist in solution. At equilibrium, the reaction is still occuring in both directions at the same rate. Such is the reaction between nitric acid and gold.
Au (s) + 3 NO3- (aq) + 6 H+ (aq) ↔ Au3+ (aq) + 3 NO2 (g) + 3 H2O (l)
In this equation, solid gold reacts with the nitrate (NO3-) ion. Nitric acid is a powerful oxidizer, which means it cause other chemicals to lose electrons. When the atoms of gold lose electrons, they become positively-charged ions that easily enter solution. But at equilibrium, this reaction leans very far to the left side of the equation — only the tiniest quantities of gold are dissolved, because only a miniscule amount of Au3+ can exist in solution.
Enter hydrochloric acid. Hydrochloric acid is capable of dissolving many types of metals, but it won't react at all with gold. But it will react with the positively-charged Au3+ ions:
Au3+ (aq) + 4 Cl- (aq) → AuCl4- (aq)
What happens here is that chloride ions are reacting with gold ions to form chloraurate ions. What that means is that the gold ions are removed from solution, and thus the first reaction, the one that dissolved the gold, can't proceed in reverse. That drives the reaction towards the right — it can only occur in one direction because the reaction's products are being stolen away. That's how aqua regia works: each acid performs a different function, and between the two of them, they manage to successfully dissolve large quantities of gold. A considerably more complex process results when platinum is dissolved in aqua regia, though ultimately it still boils down to the same thing — each acid performs a different task and thus they cooperate to dissolve the metal. It's easy to see why aqua regia had a special mystique among alchemists.
Abu Musa Jabir ibn Hayyan, also known by his Latinized name, Geber, was one of the early Muslim alchemists; he lived from approximately 721 to 815. His background is uncertain; he was most likely born in Iran and may have been Persian, like a number of other prominent Muslim scientists. Europeans have occasionally referred to him "the father of Arab chemistry". Though he was a mystic and a Sufi, he was one of the earliest pioneers of systematic experimentation, making him an important figure in the history of science.
I won't explore the breadth of his contributions to alchemy, though many instruments he invented, such as the alembic, are still in use today. Among his other work, he combined sulfuric acid with various salts and in that way discovered many other strong acids. Common salt could be used to produce hydrochloric acid, while saltpeter yielded nitric acid, and it was when he combined those two materials that he invented aqua regia. The ability to dissolve gold seemed nothing short of miraculous to alchemists, as it seemed to presage the development of the philosopher's stone, the long-sought ingredient that would transmute base metals into gold. Though of course neither he nor the other alchemists ever succeeded at this — and neither did Geber succeed in creating life from inanimate chemicals, one of his other long goals — his contributions to alchemy, and even to the far more recent science of chemistry, make him an important figure in history.
Dissolving a Nobel Medal
During World War II, Niels Bohr's Institute of Theoretical Physics in Copenhagen was something of a safe haven for Germany's Jewish scientists. Two of them, Max von Laue and James Franck, took their medals with them when they fled Germany and hid them at Bohr's laboratory. Bohr and George de Hevesy, a Hungarian chemist, conspired together to hide the medals after Denmark was invaded. They considered burying them (both to safeguard them from Nazi laws taking possession of gold and to protect the identities of the two scientists) but decided that burial wouldn't be certain enough to protect them.
Instead, as the invading troops entered the streets of Copenhagen, de Hevesy dissolved the two medals in aqua regia and left them on a shelf, dissolved, for the duration of the war. They even survived thorough searches of the laboratory by Nazi soldiers. Afterwards, de Hevesy recovered the gold from solution, and the Nobel Foundation recoined the two medals made from the same gold.