The Collaborative International Dictionary of English v.0.48:

Sulphion \Sul"phi*on\, n. [Sulpho- + ion.] (Chem.)
   A hypothetical radical, SO4, regarded as forming the acid
   or negative constituent of sulphuric acid and the sulphates
   in electrolytic decomposition; -- so called in accordance
   with the binary theory of salts. [Written also sulphione.]
   [1913 Webster]