"Even before the appearance of The Sceptical Chemist there was a growing conviction that the old hypotheses as to the essential nature of matter were inadequate and misleading. ...[T]he four "elements" of the Peripatetics had become merged into the tria prima—the "salt," "sulphur," and "mercury"—of the Paracelsians. As the phenomena of chemical action became better known... the conception of the tria prinui, as understood by Paracelsus and his followers, was incapable of being generalised into a theory of chemistry. Becher, while clinging to the conception of three primordial substances as making up all forms of matter, changed the qualities hitherto associated with them. According to the new theory, all matter was composed of a mercurial, a vitreous, and a combustible substance or principle, in varying proportions, depending upon the nature of the particular form of matter. When a body was burnt or a metal calcined, the combustible substance—the terra pinguis of Becher—escaped."