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 [Note: Authorized Version.] . In Proverbs 25:20 the effect of songs on a heavy heart is compared to the action of vinegar upon ‘nitre’ (RVm [Note: Revised Version margin.] ‘soda’). Vinegar has no effect upon saltpetre, but with carbonate of soda it produces effervescence. In Jeremiah 2:22 ‘nitre’ (RV [Note: Revised Version.] ‘lye’) is referred to as a cleansing agent. Here, again, natron rather than modern nitre suits the connexion.