The Collaborative International Dictionary of English v.0.48:

Nitrum \Ni"trum\, n. [L., natron. See Niter.] (Old Chem.)
   Niter.
   [1913 Webster]

   Nitrum flammans [L., flaming niter] (Old Chem.), ammonium
      nitrate; -- probably so called because it deflagrates when
      suddenly heated.
      [1913 Webster]