Coloured portrait of the British chemist Robert Boyle, 1627-1691. Boyle was a strong supporter of the Galilean experimental method and the atomic theory. In 1661 with the publication of The Sceptical Chemist he established chemistry as a separate science. In his book he gave the definition of an element as a substance that could not be broken down into simpler substances. He also abandoned the Aristotelian theory of the four elements, fire, air, water and earth as the basic constituents of the Universe. Boyle is famous for the formulation of Boyle's law which stated that for a gas at a constant temperature the product of its pressure and volume is a constant.