ferrous sulphate

from The Collaborative International Dictionary of English v.0.48

Copperas \Cop"per*as\, n. [OE. coperose, F. couperose, fr.
   (assumed?) L. cuprirosa, equiv. to G. cha`lkanqos, i. e.
   copper flower, vitriol. See {Copper} and {Rose.}]
   Green vitriol, or sulphate of iron; a green crystalline
   substance, of an astringent taste, used in making ink, in
   dyeing black, as a tonic in medicine, etc. It is made on a
   large scale by the oxidation of iron pyrites. Called also
   {ferrous sulphate}.
   [1913 Webster]

   Note: The term copperas was formerly synonymous with vitriol,
         and included the green, blue, and white vitriols, or
         the sulphates of iron, copper, and zinc.
         [1913 Webster] copper-bottom