sulphur supposed to be present in this sort of mercury might be purged away. [23] _Of the Sum of Perfection_ (see _The Works of Geber_, translated by Richard Russel, 1678, pp. 69 and 70). [24] _Of the Sum of Perfection_ (see _The Works of Geber_, p. 156). [25] See _The Works of Geber_, p. 160. This view was also held by other alchemists. [26] _The New Chemical Light_, Part II., _Concerning Sulphur_ (see _The Hermetic Museum_, vol. ii. p. 151). [27] See _The Golden Tract concerning the Stone of the Philosophers_ (_The Hermetic Museum_, vol. i. p. 17). The sulphur-mercury theory of the metals was held by such famous alchemists as Roger Bacon, Arnold de Villanova and Raymond Lully. Until recently it was thought to have originated to a great extent with the Arabian alchemist, Geber; but the late Professor Berthelot showed that the works ascribed to Geber, in which the theory is put forward, are forgeries of a date by which it was already centuries old (see § 32). Occasionally, arsenic was regarded as an elementary principle (this view is to be found, for example, in the work _Of the Sum of Perfection_, by the pseudo-Geber), but the idea was not general. The Sulphur-Mercury-Salt Theory. § =18.= Later in the history of Alchemy, the mercury-sulphur theory was extended by the addition of a third elementary principle, salt. As in the case of philosophical sulphur and mercury, by this term was not meant common salt (sodium chloride) or any of those substances commonly known as salts. “Salt” was the name given to a supposed basic principle in the metals, a principle of fixity and solidification, conferring the property of resistance to fire. In this extended form, the theory is found in the works of Isaac of Holland and in those attributed to “Basil Valentine,” who (see the work _Of Natural and Supernatural Things_) attempts to explain the differences in the properties of the metals as the result of the differences in the proportion of sulphur, salt, and mercury they contain. Thus, copper, which is highly coloured, is said to contain much sulphur, whilst iron is supposed to contain an excess of salt, &c. The sulphur-mercury-salt theory was vigorously championed by Paracelsus, and the doctrine gained very general acceptance amongst the alchemists. Salt, however, seems generally to have been considered a less important principle than either mercury or sulphur.