What It Actually Is

Aqua Fortis is identified as nitric acid, HNO3, an identification secure enough that historians of chemistry treat it as one of the least contested named acids in the alchemical repertoire, its selective dissolving behavior toward silver over gold following directly from gold's exceptional chemical nobility, its strong resistance to oxidation, compared to nearly every other common metal. The acid's origin in distilling saltpetre is itself a reliable chemical clue supporting the identification, since heating a nitrate salt with a strong acid is exactly the modern laboratory method for liberating nitric acid, and the correspondence between the historical recipe and the modern preparation is close enough that no real scholarly dispute exists over what substance is meant.

What later chemistry added was an explanation, rather than a revision, of why the acid behaved as it did: the specific electrochemical nobility of gold, and the ready solubility of silver nitrate, were understood only once electrochemistry gave a theoretical account of metal reactivity that the alchemical assayers had already exploited empirically for centuries. The assayer's test itself, applying aqua fortis to a suspect gold object and watching whether it dissolved, remained in practical use well into the era of modern chemistry, a rare case of an alchemical-era diagnostic procedure surviving essentially unchanged because the chemistry behind it was correctly understood empirically from the start.