54 PROCEEDINGS OF THE AMERICAN ACADEMY 



On the supposition that the zinc used in the reduction of the silver 

 is the source of the lead in the American coin, it is easy to calculate 

 the amount of lead which would thus find its way into the coin, since 

 the quantity of zinc used in reducing a given weight of silver, and the 

 per cent of lead which that zinc may be expected to contain, are both 

 known quantities. Professor Booth* says that an excess of zinc is 

 required to insure total and rapid reduction, and Wilson f states, that 

 two equivalents of zinc are used, in practice, for each equivalent of 

 silver. Our memoir, already cited, gives the per cents of lead found 

 in two specimens of Vieille Montague zinc. The standard of the 

 American silver coin is -^jy silver and -^ copper, and the weight of fifty 

 cents' worth of this alloy, in either half-dimes, dimes, quarters, or a 

 half-dollar, has been 192 grains := 12.433 grammes, since the year 

 1853.J 



Fine silver in the half-dollar, . . . 11.190 gram. 

 Zinc used in reducing 11.19 gi'am. silver, . 6.742 " 



Amount of lead in 6.742 gram, zinc, if the zinc 



contains 0.292 per cent of lead, § . . 0.0197 " 

 Amount of lead in 6.742 gram, zinc, if the zinc 



contains 0.494 per cent of lead, § . . . 0.0333 « 



If zinc of the best quality (containing 0.292 per cent, of lead) had 

 been used, the silver coin would have contained 0.158 per cent of 

 lead ; if the second quality (containing 0.494 per cent of lead) has 

 been employed, the coin may contain 0.268 per cent of lead. Between 

 these two limits all our determinations of lead in American silver will 

 be found to lie. 



In offering this explanation of the occurrence of lead in American 

 silver coin, we would by no means affirm that the zinc is the exclusive 

 source of this impurity, for it is not at all improbable that a portion of 

 the lead is derived from the leaden vats in which the reduction of the 



determined, exhibited a slightly yellowish-light-brown color, nothing similar to 

 which occurred in any of the other experiments. A slight black residue remained 

 when this silver was dissolved in nitric acid, and a trace of gold was detected in 

 the residue described on p. 59. 



* Loc. cit. t Loc. cit. 



X Brightly's Dig. Laws U. S., for Standard, Title Coinage, § 3 ; for Weights, 

 Title Coinage, § 13. 



§ See our Memoir in Mem. Amer. Acad. [n. s.], VIII. 61, Table I. 



