Practically... there was little difference in the application of these — Tria prima

"The doctrine of the four elements seems to have continued undisputed till the time of the alchemists. These men, better acquainted than the ancient philosophers with the analysis of bodies, became convinced of the inadequacy of that doctrine to explain all the phenomena which were presented to their view. Hence they substituted in its stead a theory of their own; namely, that all bodies are composed of three elements, salt, sulphur, and mercury, which they distinguished by the appellation of the tria prima. To these principles, which were embraced by succeeding writers, Paracelsus added two more, phlegm and caput mortuum."

"Alchemical theory was essentially static throughout the medieval period. ...Paracelsus was the herald of a new era, an era of . His contribution to alchemical theory lay in the addition to sulphur and mercury of a third principle, which he called salt. Materially this was recognised as the principle of uninflammability and fixidity. ...[T]he tria prima, or three hypostatical principles could be interpreted in either a material or a spiritual sense. In the words of Paracelsus himself: Know, then, that all the seven metals are born from a threefold matter... Mercury is the spirit, Sulphur is the soul, and Salt is the body... the soul... unites those two contraries, the body and spirit, and changes them into essence. ...similar to the material effect of the liquid menstruum, or Hermetic Stream, in uniting sophic sulphur and sophic mercury to produce the Philosphers Stone."

"Boyle attacked this hypothesis in his Sceptical Chemist, and several of his other publications: proving that under each of the terms salt, sulphur, mercury, phlegm, and earth are comprehended substances of very different properties; that all bodies are not composed of these principles; and that the principles themselves are not elements, but compounds."

"In conformity with this theory they maintained, that all bodies may be decomposed by means of fire into these three principles; the salt remains behind fixed, the sulphur takes fire, and the mercury flies off in the form of smoke. The phlegm and caput mortuum of Paracelsus were the water and earth of the ancient philosophers."

"Mercury, sulphur, and salt were not three matters, but one, derived from the ."

"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."