Source : Webster's Revised Unabridged Dictionary (1913)

Thallous \Thal"lous\, a. (Chem.)
   Of or pertaining to thallium; derived from, or containing,
   thallium; specifically, designating those compounds in which
   the element has a lower valence as contrasted with the
   {thallic} compounds. [Written also {thallious}.]