thallous--prophetes.ai

thallous, a. Chem.
  (ˈθæləs)
  [f. thall-ium + -ous: cf. aluminous, tantalous.]
  = thallious.

1888 Encycl. Brit. XXIII. 220/1 Thallic salts are related to thallous pretty much as manganic are to manganous... Thallous chloride.

Oxford English Dictionary