ETYM French nitre, Latin nitrum native soda, natron, Greek; cf. Arabic nitűn, natrűn natron. Related to Natron.
Or saltpeter; Potassium nitrate, KNO3, a mineral found on and just under the ground in desert regions; used in explosives. Niter occurs in Bihar, India, Iran, and Cape Province, South Africa.
The salt was formerly used for the manufacture of gunpowder, but the supply of niter for explosives is today largely met by making the salt from nitratine (also called Chile saltpeter, NaNO3). Saltpeter is a preservative and is widely used for curing meats.
Potassium nitrate; Chile saltpeter.
Saltpeter (potassium nitrate).

Treat with nitric acid.