He attended the meetings of a religious sect at the Leathersellers' Hall in London. It is possible that it was related to the Society of Friends, as he published An apology for the innocency and justice of the Quakers cause. And a short elucidation of their principles in 1674. From 1687 to 1690, a period of religious and political tension in the reign of James II of England, Salmon is said to have left England and travelled in New England and the Caribbean. After his return, he wrote books on baptism and transubstantiation which are listed in A Descriptive Catalogue of Friends' Books. A Discourse against Transubstantiation (1690) was written in the form of a dialogue between a Protestant and a Catholic. A Discourse on Water Baptism appeared in 1700. Salmon's works frequently included portraits, with surrounding details that vary between editions. George Vertue lists several portraits of Salmon in A catalogue of engravers (1782). Robert White is credited with a portrait of "William Salmon, M.D." in 1700, while a painter named Burnford "is known only by a print of William Salmon, chymist, 1681." A third portrait by William Sherwin, dating to 1670, is included in Portraits of doctors & scientists in the Wellcome Institute of the History of Medicine. Other engravings include one by Michael Vandergucht after White. Works Salmon's published works covered an incredibly wide range of topics, including pharmacology, medicine, surgery, alchemy, chiromancy, astrology, almanacs, botany, cooking, and art. In part, he was able to publish so prolifically because large sections of his texts were "copied, translated, abridged, enlarged and compiled from the texts of others". Salmon openly acknowledged that much of his work was derivative, stating "we have scrutinized the best Authors, to many of which we have been very much beholden", with an extensive list of members of the Mathematical, Medical, and Chyrurick Tribes, as well as Anatomists, chymists, and a multitude of others. Even by the standards of his time, he was criticized for failing to individually credit the sources from which specific materials were taken. In addition, instead of taking a specific philosophical position and siding with one of the competing schools of medical thought, Salmon created "a compendium of everything". The publication history of his books is complicated, as subsequent editions of a title often added more and more material. For example, the 1701 edition of Polygraphice was almost three times the