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]