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BOOK VII.

silver, combined with a certain portion of lead, into the cupel, in which waythe assay is rendered erroneous. Then take the beads out of the cupel andclean them of dross. If neither depresses the pan of the balance in which itis placed, but their weight is equal, the assay has been free from error ; butif one bead depresses its pan, then there is an error, for which reason theassay must be repeated. If the bes of coin contains but seven unciae ofpure silver it is because the King, or Prince, or the State who coins the money,has taken one uncia, which he keeps partly for profit and partly for theexpense of coining, he having added copper to the silver. Of all thesematters I have written extensively in my book De Precio Metallorum etMonetis.

We assay gold coins in various ways. If there is copper mixed withthe gold, we melt them by fire in the same way as silver coins ; if there issilver mixed with the gold, they are separated by the strongest aqua valens ;if there is copper and silver mixed with the gold, then in the first place, afterthe addition of lead, they are heated in the cupel until the fire consumes thecopper and the lead, and afterward the gold is parted from the silver.

It remains to speak of the touchstone 37 with which gold and silver aretested, and which was also used by the Ancients. For although the assay madeby fire is more certain, still, since we often have no furnace, nor muffle, norcrucibles, or some delay must be occasioned in using them, we can alwaysrub gold or silver on the touchstone, which we can have in readiness.Further, when gold coins are assayed in the fire, of what use are they after-ward ? A touchstone must be selected which is thoroughly black and freeof sulphur, for the blacker it is and the more devoid of sulphur, the better it

37 Historical Note on Touchstone ( Coticula. Interpretatio, — Goldstein). Theophrastusis, we believe, the first to describe the touchstone, although it was generally known to theGreeks, as is evidenced by the metaphors of many of the poets,—Pindar, Theognis, Euripides, etc.The general knowledge of the constituents of alloys which is implied, raises the question asto whether the Greeks did not know a great deal more about parting metals, than has beenattributed to them. Theophrastus says (78-80) : “ The nature of the stone which tries“ gold is also very wonderful, as it seems to have the same power with fire ; which is also“ a test of that metal. Some people have for this reason questioned the truth of this power“ in the stone, but their doubts are ill-founded, for this trial is not of the same nature or“ made in the same manner as the other. The trial by fire is by the colour and by the" quantity lost by it; but that by the stone is made only by rubbing the metal on it; the“ stone seeming to have the power to receive separately the distinct particles of different“ metals. It is said also that there is a much better kind of this stone now found out, than“ that which was formerly used ; insomuch that it now serves not only for the trial of refined“ gold, but also of copper or silver coloured with gold ; and shows how much of the“ adulterating matter by weight is mixed with gold ; this has signs which it yields from“ the smallest weight of the adulterating matter, which is a grain, from thence a colybus,“ and thence a quadrans or semi-obolus, by which it is easy to distinguish if, and in what“ degree, that metal is adulterated. All these stones are found in the River Tmolus ; their“ texture is smooth and like that of pebbles; their figure broad, not round; and their“ bigness twice that of the common larger sort of pebbles. In their use in the trial of metals“ there is a difference in power between their upper surface, which has lain toward the sun,“ and their under, which has been to the earth ; the upper performing its office the more“ nicely; and this is consonant to reason, as the upper part is dryer ; for the humidity of“ the other surface hinders its receiving so well the particles of metals ; for the same reason“ also it does not perform its office as well in hot weather as in colder, for in the hot it emits“ a kind of humidity out of its substance, which runs all over it. This hinders the metalline“ particles from adhering perfectly, and makes mistakes in the trials. This exudation of a“ humid matter is also common to many other stones, among others, to those of which“ statues are made ; and this has been looked on as peculiar to the statue.” (Based on