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Lapis Lydius, Touchstone, Assaying

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LAPIS LYDIUS.                                   211
LAPIS LYDIUS: Touchstone: Assaying.
Bacchylides, the lyric poet (b.c. 450), introduces in a simile, and Theophrastus (47) minutely describes this most ancient of the methods of assaying the precious metals : "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 ; for 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 the old one has been discovered, capable of testing not only fine gold, but also that alloyed with copper or silver, 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 (1-1/2 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 smooth, and like that of a counter, flat, not spherical, but their size about twice that of the biggest counter used. For the pur­pose of assaying, there is a difference between the upper part of the stone which has been exposed to the sun, and the underside, the latter being the better test. This is a necessary consequence, inasmuch as the upper side is of a drier nature : for moisture is an impediment to its taking off the metal ; hence in very hot weather the stone docs not act so well, for it then emits a certain dew out of itself, by reason whereof it becomes slippery."
ρ 2
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