Nitre

(Prov. 25:20; R.V. marg., "soda"), properly "natron," a
substance so called because, rising from the bottom of the Lake
Natron in Egypt, it becomes dry and hard in the sun, and is the
soda which effervesces when vinegar is poured on it. It is a
carbonate of soda, not saltpetre, which the word generally
denotes (Jer. 2:22; R.V. "lye").