Of eretria, or Eretrian[1] earth, there are also the same number of varieties; one white, and the other of an ashy colour, this last being preferred in medicine. To be good, this earth should be of a soft consistency, and when rubbed upon copper it should leave a violet tint. The virtues of cretria in a medicinal point of view, and the methods of using it, have been already mentioned[2] in our description of the pigments.