The Collaborative International Dictionary of English v.0.48:

Tetrathionic \Tet`ra*thi*on"ic\, a. [Tetra- + thionic.] (Chem.)
   Of, pertaining to, or designating, a thionic derivative,
   H2S4O6, of sulphuric acid, obtained as a colorless,
   odorless liquid.
   [1913 Webster]