The art and science of alchemy, which thrived from around 200BC until the 18th century, was founded on rational principles.

Plato and Aristotle had developed a theory of matter that held everything to be made of earth, air, fire and water.

Each of those in turn combined two of the primary qualities: hot, cold, wet and dry. Earth is cold and dry, air is hot and wet, fire is hot and dry, water is cold and wet.

When fire loses its heat, it becomes earth - in the form of ash; when water is heated, it becomes air - in the form of steam. In principle, anything can therefore be turned into anything else by stripping it down to its basic components, then remixing them with a little heating, chilling, wetting or drying.

But to do it properly, it was necessary to have the Philosopher's Stone, which has the power to turn all things to gold. 'I have seen and handled more than once the Stone of the Philosophers,' said J.B. van Helmont. 'In colour it was like powder of saffron but heavy and shining even as powdered glass.' Van Helmont, incidentally, was a noted chemist who invented the word 'gas'.

The Stone, however, did not only turn base metal to gold. It was also believed to have the power of giving eternal life and transforming a person from earthly impurity into heavenly perfection.

Here, however, we must make a distinction between exoteric (outward) and esoteric (hidden) alchemy. The former was concerned with the practical business of finding the Stone, concocting an Elixir of Life, making gold and living for ever.

Esoteric alchemy, however, was a mystical and religious concept linked to the belief that only the pure of soul would be granted the divine grace needed to discover the Philosopher's Stone. Thus, the first objective became not so much transmuting metal into gold, but transmuting the sinful towards purity.

There are 15 alchemists listed in the British Dictionary of National Biography, of whom the following deserve special mention:

Sir Thomas Ashton and Sir Edmund de Trafford who, in 1446, were given special privileges by Henry VI to continue their experiments. The King also forbade any of his subjects from molesting them.

Thomas Charnock (1526-81), who was bequeathed the secret of the Philosopher's Stone by an alchemist in Salisbury but was frustrated when his equipment perished in a fire in 1555. Two years later he again believed he was on the verge of success when he was called up for the relief of Calais.

Edward Kelley (1555-95), who claimed to have discovered the Philosopher's Stone among the ruins of Glastonbury Abbey. He had his ears cropped in the pillory at Lancaster after a conviction for forgery.

John Dee (1527-1608), who employed Kelley, never noticed the latter's mutilated ears (which were hidden beneath a skull cap). Dee sent Queen Elizabeth a piece of gold that he said he had made from metal cut from a warming pan, and gave his son quoits supposedly made from transmuted gold.

John Damian was court alchemist to James IV of Scotland. In 1507, he built himself a pair of wings and tried to fly from the ramparts of Stirling Castle but fell to the ground and broke a leg. He blamed the feathers he had used, which came from barnyard fowl unaccustomed to flying. Despite this failure, James IV still paid £15.16s.0d for his alchemist's gown of damask and £4 for his velvet socks.

Alchemy began to fade when Robert Boyle published The Sceptical Chymist in 1661, in some ways fulfilling Isaac Newton's prediction that science would lead away from alchemy and theology towards materialism and atheism.

Its scientific respectability, however, was partly restored in the 1950s when Carl Jung represented the whole of alchemy as a basis for depth psychology.

More recently, Beethoven's piano sonata Opus 110 has also been compared in structure to the symbolic patterns of alchemy, while J.K. Rowling's Opus 1, Harry Potter and the Philosopher's Stone, was called Harry Potter and the Sorcerer's Stone in the American edition, presumably because American children were thought unlikely to know what a philosopher was.

The French alchemist Nicholas Flamel, whose search for the Philosopher's Stone inspired parts of Rowling's story, lived from 1330 to 1418. The house he built in 1407 is now considered the oldest building in Paris and is the site of a restaurant, the Auberge Nicholas Flamel.

In 2009, a set lunch there was priced at 18.50 euros for two courses with wine and coffee, or 25 euros for three courses.