There is reason to believe, as stated already, that many of the published works ascribed to these learned men are the work of obscure individuals who traded on their fame. What may with certainty be credited to them serves to show that their theoretical opinions had much in common. They all regarded the transmutation of metals and the existence of the philosopher’s stone as facts which could not be controverted. They followed Geber in assuming that all the metals were essentially compound in their nature, and consisted of the essence or “element” of mercury, united with different proportions of the essence or “element” of sulphur. The alchemists were the professional chemists of their time, and many of them were practising physicians. Indeed, professional chemistry may be said to have originated out of the practice of physic. As the number of chemical products increased and their value in therapeutics became more and more appreciated, there arose another school of alchemists, whose energies were devoted, not to the transmutation of metals—which, however plausible as a belief, seemed hopeless of achievement—but to the more immediate practical benefits which it was recognised must follow from the closer association of chemistry and medicine. This school came to be known as the iatro-chemists. As their doctrines exercised a great influence upon the development of chemistry, it will be desirable to treat of them and their professors in a special chapter. CHAPTER IV THE PHILOSOPHER’S STONE