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Lapis Lydius, Touchstone, Assaying

Lapis Lydius, Touchstone, Assaying Page of 384 Lapis Lydius, Touchstone, Assaying Text size:minus plus Restore normal size   Mail page  Print this page
LAPIS LYDIUS.
153
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 does not act equally well, for it then emits a certain dew out of itself, by reason whereof it becomes slippery."
Pliny (xxxiii. 43) refers to this description by Theo-phrastus, adding that the Touchstone, besides its Latin name "coticula," was called the Lydian stone, and the Herculean. He further states, what seems incredible, that experienced assayers, by rubbing off a piece of any ore on this stone, " cum e vena ut lima rapuerunt experimentum," were enabled to declare without mistake how much gold, silver, or copper it contained, within one scruple, " scriptu-lari differentia." From its use the stone received the epithet of " Paragonius " or comparative, whence by a singular transition of ideas comes our ' Paragon ' in the sense of ' Pattern of excellence.'
The present Touchstone is a black Jasper of a somewhat coarse grain, and the best pieces come from India. The Italian goldsmiths employ it in the following ingenious manner. They have a set, strung on a ring, of 24 " needles," little bars of gold, each of a known and marked standard from one carat up to twenty-four (or fine). Taking the gold to be assayed, they rub it on the stone ; by the side of the streak it leaves, they rub the needle which seems to the eye the nearest in quality. Next they pour " aqua regia " on the two streaks, and if the solvent produces the same effect on each, it proves that the gold in the piece and in the needle is of the same standard. If there be a
Lapis Lydius, Touchstone, Assaying Page of 384 Lapis Lydius, Touchstone, Assaying
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