NITRE, in its modern usage, denotes saltpetre, nitrate of potash, but the nitron or nitrum of the ancients was a different substance, natron, carbonate of soda. 'Nitre' occurs twice in AV. In Pr 25:20 the effect of songs on a heavy heart is compared to the action of vinegar upon 'nitre' (Revised Version margin 'soda'). Vinegar has no effect upon saltpetre, but with carbonate of soda it produces effervescence. In Jer 2:22 'nitre' (RV 'lye') is referred to as a cleansing agent. Here, again, natron rather than modern nitre suits the connexion.