Spirit of Hartshorn is identified as an aqueous solution of ammonia, NH3 dissolved in water, an identification supported both by its preparation, the dry distillation of nitrogen rich animal matter being a genuine and still recognized route to ammonia, and by its characteristic properties, alkalinity, a sharp irritating smell, and effectiveness as a smelling salt, all of which match ammonia's known chemistry closely.
Ammonia's status as the shared chemical basis of both spirit of hartshorn and sal ammoniac, despite their very different preparations, one from animal horn and one from soot and dung, was itself an important early clue that helped eighteenth century chemists recognize ammonia as a single definite substance obtainable by several different routes rather than a family of vaguely related volatile alkalis, a conceptual step that mirrors, on the alkaline side of chemistry, the parallel recognition of oil of vitriol as a single acid obtainable from several vitriols. Spirit of hartshorn's continued sale as smelling salts well into the twentieth century, long after its animal-horn origin had been replaced by cheaper industrial ammonia sources, shows how thoroughly a traditional name can outlive the specific process that first gave rise to it.