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. " 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.