Of eretria, or Eretrian1 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 mentioned2 in our description of the pigments.
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