Hastings' Dictionary of the Bible (1898)
NI'TRE, an earthy alkaline salt, resembling and used like soap, which, separating from the bottom of the lake Natron, in Egypt, and rising to the top, is condensed by the heat of the sun into a dry and hard substance similar to the Smyrna soap, and is the soda of common earth. It is found in many other parts of the East. Vinegar has no effect upon common nitre, and of course this could not be meant by the wise man, who in Prov 25:20 says, "As he
that taketh away a garment in cold weather, and as vinegar upon nitre, so is he that singeth songs to a heavy heart." Now, as vinegar has no effect upon nitre, but upon natron or soda its action is very obvious, it seems the English translation should have been "natron." In Jer 2:22 the same word again is improperly used: "For though thou wash thee with nitre, and take thee much soap, yet thine iniquity is marked before me, saith the Lord God."
The alkaline earth natron is obviously designed in this passage. It is found as an impure carbonate of soda on the surface of the earth in Egypt and Syria, and is also native in some parts of Africa in hard strata or masses, and is called trona being used for the same purposes as the barilla of commerce.