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.