"Paracelsus made an important contribution to chemical theory. He extended the sulphur-mercury theory of the Islamic chemists by adding a third principle... salt. Thus, when wood burned, the combustible component was identified with sulphur, the volatile component with mercury and the ashes... with salt. The composition of all substances could be expressed in terms of these three principles, or tria prima. As in previous theories... [these] were not... common materials... but rather... essential qualities."