Melvyn Bragg and guests discuss the chemical elements. The aim and challenge in chemistry, according to the Encyclopaedia Britannica, is the understanding of the complex materials which constitute everything in existence since the Big Bang, when the whole universe emerged out of the two elements of hydrogen and helium. For Aristotle there were four elements: Earth, Air, Fire and Water. Now there are one hundred and eight, sixteen of which are produced artificially, and none of which figure in Aristotle's original four. But they are all still elements - defined as substances which cannot be broken down, the building blocks of all life.Today we have the key to understanding these elements, the Periodic Table, which is a pattern embedded in nature and was miraculously discovered in a dream. With Paul Strathern, former lecturer in philosophy and science, Kingston University and author of Mendeleyev's Dream: The Quest for the Elements; Dr Mary Archer, Visiting Professor of Chemistry at Imperial College, London; John Murrell, Emeritus Professor of Chemistry, University of Sussex.