For about two thousand years purple was the colour of emperors, because the good purple came from sea snails, crushed by the thousands for a single gram of dye. In 1856 an eighteen-year-old named William Perkin, home for the Easter holidays, was trying to make quinine out of coal tar and got a black sludge instead. Cleaning out the flask with alcohol, he noticed the rinse had turned a strong purple, and he didn’t pour it away. Within a few years London was wearing mauve, made from the leftovers of the gasworks, and the colour of emperors was on ordinary people’s ribbons. I like that the find was in the washing-up: the experiment had already failed, and he was still looking.