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1911 Encyclopedia Britannica
the name given to naturally occurring potassium nitrate; "cubic nitre" is sodium nitrate. The word is adapted from Lat. nitrum, which is itself adapted from Gr. virpov. These words were originally applied to the naturally occurring sodium carbonate; the connexion with potassium nitrate (sal petrae or sal petrosum) may be traced to Raimon Lull's name sal nitri, which substance, however, he distinguished from nitrum. In the 16th century the ancient nitrum became altered to natron, a term still used for native sodium carbonate, while nitrum, and its adaptation nitre, were retained for potassium nitrate or saltpetre.
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Chisholm, Hugh, General Editor. Entry for 'Nitre'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/n/nitre.html. 1910.