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152 NATURAL HISTORY OF GEMS.
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LAPIS LYDIUS: Touchstone: Assaying: Alloys: Foils.
Bacchylides, the lyric poet, who flourished b.c. 450,
introduces in a simile,* and Theophrastus minutely describes this most
ancient of all methods of assaying the precious metals (47) : "
Wonderful again is the property of that stone used for testing gold,
for it appears in fact to possess the same power as fire, for that also
acts as a test. For which reason some are puzzled about it in
consequence of their not viewing the question in a proper light. For
the stone does not really try the gold in the same manner as the fire;
because the latter acts as a test by changing and altering the colour
of the metal, whereas the former tests it by friction. For it has the
power, as it appears, to distinguish the nature of each metal, the gold
and the alloy ; and it is said that lately a much better kind of
Touchstone than fh old one has been discovered, capable of testing not
only gold after the refining, but also copper containing gold, as well
as silver (similarly auriferous), and of showing how much alloy has
been mixed with the standard of the gold coin. The assays are taken
from the smallest quantity, for the least weight is a grain of barley,
the next the collybus (1-1/4 gr.), then the quarter, then the half-obol
(5 gr.) : from which they ascertain the amount of the alloy—all such
Touchstones are found in the river Tmolus. Their form is
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