For fifteen hundred years, from the workshops of Alexandria to the laboratories of London, alchemists chased one thing above all others: the Philosopher's Stone.

It was said to be a red powder — or sometimes a waxy red substance — that could transmute a base metal like lead into pure gold, and, dissolved in wine, cure any illness and grant its owner extremely long life.

To make it, the alchemist had to take a prime matter through four stages, each named after a colour: nigredo the black, albedo the white, citrinitas the yellow, and rubedo the red.

Nobody ever produced it — but in trying, alchemists invented distillation, discovered phosphorus, mapped acids and bases, and built the first proper laboratories.

The Philosopher's Stone was chemistry's north star.

Chemistry never reached it, but it discovered almost everything else on the way.