From nathar; mineral potash (so called from effervescing with acid) -- nitre.
see HEBREW nathar
H5427. nether
נֶ֫תֶר noun [masculine] natron, or carbonate of soda, a mineral alkali (Late Hebrew = Biblical Hebrew; Aramaic נִתְרָא, ; compare Greek νίτρον, λίτρον, Latin nitrum (see Lexicons)); — בַּנֶּתֶר אִםתְּֿכַבְּסִי Jeremiah 2:22 though thou wash thyself with natron (figurative; "" בֹּרִית; on use of 'nitrum' for handwashing among Greeks see MeinekeFragm. Comicorum ii. 638); עַלנָֿ֑תֶר חֹמֶץ Proverbs 25:20 (figurative of the incompatible).