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Smith's Bible Dictionary
Nitre. Mention of this substance is made in Proverbs 25:20 - "and as vinegar upon nitre" - and in Jeremiah 2:26. The article denoted is not that which we now understand by the term, nitre, that is, nitrate of Potassa - "saltpetre" - but the nitrum of the Latins, and the natron, or native carbonate of soda, of modern chemistry. Natron was, and still is, used by the Egyptians for washing linen. The value of soda in this respect is well known. This explains the passage in Jeremiah. Natron is found in great abundance, in the well-known soda lakes of Egypt.
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Smith, William, Dr. Entry for 'Nitre'. Smith's Bible Dictionary. https://www.studylight.org/dictionaries/eng/sbd/n/nitre.html. 1901.