Spirit of Salt

Spirit of Salt was made by distilling common salt with oil of vitriol, heating the mixture in a retort so that the sulfuric acid displaced the more volatile acid component of the salt, which distilled over as a sharp, fuming, colorless to yellowish liquor. Compared to aqua fortis, spirit of salt was a comparatively gentle acid toward most metals, dissolving some but leaving others largely untouched, and it did not share aqua fortis's dramatic selectivity for silver over gold. Its most useful and most reliably documented property was its reaction with dissolved silver: adding spirit of salt to a silver solution throws down a heavy white, curd like precipitate, silver chloride, a reaction alchemists used as a recognition test for silver in solution and that later became foundational to analytical chemistry's qualitative tests for the halide ions.

Combined with common salt itself, or used to prepare sal ammoniac and other chloride salts, spirit of salt was a steady workhorse reagent rather than a spectacular one, valued for reliability more than for the drama of its reactions. It was also, alongside the other two mineral acids, part of the standard trio, vitriolic, nitrous and marine, that eighteenth century chemists used as the basic classification of the strong acids before a more systematic nomenclature replaced the old names.

Spirit of Salt is hydrochloric acid, HCl, an identification about as secure as any in this atlas, since the substance's preparation from common salt, its characteristic reaction with silver to form an insoluble white chloride, and its general behavior all match the modern compound with no meaningful gap between the historical description and the chemistry. Its later name, muriatic acid, coined once eighteenth century chemists began systematizing acid nomenclature around the salt, muria in Latin, from which each acid was derived, made the connection to common salt even more explicit than the older name spirit of salt had, and the compound kept that name in ordinary commercial and industrial use well into the twentieth century even after hydrochloric acid became the preferred chemical name.

There is essentially no historical ambiguity to report for this entry; the interest lies less in identifying the substance than in how consistently its recognition test, the white precipitate with silver, was used correctly by chemists across several centuries before they could explain the reaction in terms of insoluble ionic compounds. That same silver chloride precipitate reaction, recognized and used correctly across centuries of alchemical and early chemical practice, later became one of the standard qualitative tests taught in introductory analytical chemistry, a direct, traceable line from workshop observation to classroom method.