Just as the Aristotelian “elements” were qualities which, according to their degree, determined the nature of substances, so, in like manner, the specific character of a metal depended upon the relative proportion of its “sulphur” and “mercury.” These terms had no certain reference to what we to-day understand by sulphur and mercury. They denoted simply qualities. The essence or “element” of mercury conferred lustre, malleability, ductility, and fusibility, or, speaking generally, the properties which we connote as metallic; while to the essence or “element” of sulphur was to be attributed the combustibility—or, speaking generally, the alterability—of the metal by fire. By modifying the relative proportion of these constituent elements, or by purifying them from extraneous substances by the operations of chemistry, it was conceived that the several metals could be changed one into the other. To effect this purification it was necessary to add various preparations known as “medicines,” chief among which was the _Great Elixir_, or _Magisterium_, or the _Philosopher’s Stone_, by which the final transformation into the noblest of the metals could alone be achieved. The Arabic words _kímyâ_ and _iksír_ were originally synonymous and each was used to denote the agent by which the baser metals could be transmuted into silver and gold. Ultimately the former term became restricted to indicate the art of transmutation (alchemy), whereas _iksír_, or _al-iksír_, continued to denote the medium by which the transmutation was effected. By later writers the term was used to indicate a liquid preparation—the _quintessence of the philosophers_—whence we have the word _elixir_, which always means a liquid. The alchemistic theory of the compound nature and mutual relations of the metals is usually ascribed to Geber; but, although he adopted it, he distinctly states that it did not originate with him, but that he found it in the writings of his predecessors. The idea of the _stone_, the _philosophical powder_, the _grand magisterium_, the _elixir_, the _tincture_, the _quintessence_—by all of which terms the transmuting medium is known in the literature of alchemy—is probably connected with another conception respecting the origin of metals which can be traced to very early times and was prevalent throughout the Middle Ages. It was supposed of old that metals were _generated_ within the earth, as animals and plants were generated on its surface, and that something akin to a seed, or semen, was needed to initiate their formation. The great problem of alchemy was to discover this fecundating substance, as upon it depended the genesis of the perfect metal. This idea of the conception of metals runs through the literature of alchemy. It explains many allusions and much of the terminology of its writers. For example, the furnace in which the alchemist makes his projection is constantly spoken of as the _philosophical egg_.