What It Actually Is

Aqua Vitae is identified as distilled ethanol, more precisely an ethanol and water mixture whose concentration depended on how many times it had been redistilled, an identification with no real scholarly dispute attached to it since the fermentation and distillation processes described in the historical sources are essentially the same processes, using more primitive equipment, still used to make distilled spirits today.

What changed between the alchemical period and the modern understanding was not the identity of the substance but the account of why it worked and why it had limits: modern chemistry explains both the flammability of concentrated ethanol and the practical ceiling on how pure a distilled spirit can become through ordinary distillation, the azeotrope formed between ethanol and water that prevents further separation by heat alone, a limit early distillers ran into empirically without being able to say why repeated rectification eventually stopped yielding a stronger product. Reaching a genuinely anhydrous, water-free spirit required techniques beyond simple redistillation, chemical drying agents among them, that were only fully worked out well after the alchemical period, though the practical spirits distillers achieved by careful rectification alone were strong enough for essentially every purpose the period needed.