1 he last six may be in part also alteration products from all silver minerals.
The reasons for indefiniteness in determination usually lie in the failure of ancient
authors to give sufficient or characteristic descriptions. In many cases Agricola is sufficiently
definite as to assure certainty, as the following description of what we consider to be silver
glance, from De Natura Fossilium (p. 360), will indicate : " Lead-coloured rudis silver is
" called by the Germans from the word glass (glasertz), not from lead. Indeed, it has
" the colour of the latter or of galena (plumbago), but not of glass, nor is it transparent
" like glass, which one might indeed expect had the name been correctly derived. This
" mineral is occasionally so like galena in colour, although it is darker, that one who is not
" experienced in minerals is unable to distinguish between the two at sight, but in substance
" they differ greatly from one another. Nature has made this kind of silver out of a little
" earth and much silver. Whereas galena consists of stone and lead containing some silver.
" But the distinction between them can be easily determined, for galena may be ground
" to powder in a mortar with a pestle, but this treatment flattens out this kind of rudis silver.
" Also galena, when struck by a mallet or bitten or hacked with a knife, splits and breaks to
" pieces; whereas this silver is malleable under the hammer, may be dented by the teeth,
" and cut with a knife."
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