h2s2o7

from The Collaborative International Dictionary of English v.0.48

Pyrosulphuric \Pyr`o*sul*phu"ric\, a. [Pyro- + -sulphuric.]
   (Chem.)
   Pertaining to, or designating, an acid called also
   {disulphuric} acid) obtained by distillation of certain
   sulphates, as a colorless, thick, oily liquid, {H2S2O7}
   resembling sulphuric acid. It is used in the solution of
   indigo, in the manufacture of alizarin, and in dehydration.
   [1913 Webster]

from The Collaborative International Dictionary of English v.0.48

Disulphuric \Di`sul*phu"ric\, a. [Pref. di- + sulphuric.]
   (Chem.)
   Applied to an acid having in each molecule two atoms of
   sulphur in the higher state of oxidation.
   [1913 Webster]

   {Disulphuric acid}, a thick oily liquid, {H2S2O7}, called
      also {Nordhausen acid} (from Nordhausen in the Harts,
      where it was originally manufactured), {fuming sulphuric
      acid}, and especially {pyrosulphuric acid}. See under
      {Pyrosulphuric}.
      [1913 Webster]