The Collaborative International Dictionary
nitrum
Trona \Tro"na\, n. [Of Egyptian or North African origin.] (Chem. & Min.) A native double salt, consisting of a combination of neutral and acid sodium carbonate, Na2CO3.2HNaCO3.2H2O, occurring as a white crystalline fibrous deposit from certain soda brine springs and lakes; -- called also urao, and by the ancients nitrum.
Wiktionary
nitrum
n. (context chemistry obsolete English) niter