The Collaborative International Dictionary of English v.0.48:

Thomas phosphate \Thom"as phos"phate\ or Thomas slag \Thom"as
slag\ .
   Same as Basic slag, above.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Basic slag \Basic slag\
   A by-product from the manufacture of steel by the basic
   process, used as a fertilizer. It is rich in lime and
   contains 14 to 20 per cent of phosphoric acid. Called also
   Thomas slag, phosphatic slag, and odorless phosphate.
   [Webster 1913 Suppl.]