ALL, 
On the Impurities of Commercial Zinc, with special Reference to the Residue insoluble in 
Dilute Acids, to Sulphur, and to Arsenic. 
- 
By CHARLES W. ELIOT aw» FRANK H. STORER. 
(Communicated May 29, 1860.) 
WHEN common zinc is dissolved in dilute acids, as in the ordinary process of pre- 
paring hydrogen, a finely divided black substance remains undissolved, sometimes 
floating, sometimes sinking in the liquid. . The blackness and apparent lightness of 
this residue from zinc dissolved in dilute acids seem to indicate that it is carbon, which 
may perhaps account for the widely spread error that carbon is the chief admixture in 
crude zinc, as it is in cast-iron. This error has been often corrected, but the infor- 
mation on the whole subject lacks precision and certainty. The statements of the best 
authorities in regard to the impurities of zinc are for the most part loose, inaccurate, 
and sometimes contradictory.* To obtain precise and definite knowledge, if possible, 
of the chief impurities of this important metal, this investigation was undertaken. 
* The black residue was observed and experimented upon be HeLLoT (Mémoires de l'Academie des Sci- 
ences, 1735, [Mem.] p. 18, et seg.) and by De Lassone (Ibid. 1777, [Mem.] p. 11 et seq.) 
BERGMANN says, in his Physical and Chemical Essays, translated by Cullen, London, 1788, II. 321, that 
pure zinc is extremely rare, and commends the zinc brought by Grill from China as containing scarcely half of 
one per cent of lead; the zinc of Goslar is stated to contain somewhat more lead, but neither of these zines 
contained the smallest particle of iron. 
Fourcroy. For an express statement that this black powder is plumbago, see Fourcroy's Elements of 
Natural History and Chemistry, translated from the Paris edition of 1789, London, 1790, II. 289. 
Proust. “It is commonly thought that the black powder, which separates from its (zinc's) solution in sul- 
phurie acid, is charcoal or a carbide of zinc: it is nothing but arsenic, mixed with a little lead and copper, 
which the reducing power of the zinc precipitates in the metallic state." Savans Étrangers, [2.] I. 211 
(Paris, published in 1806). “Iron is found in very large quantity in it (zinc), and sometimes manganese." 
Ibid. 212. It should be remarked, that the precipitate produced by sulphuretted hydrogen, and called arsenic 
YOL. VIII. 8 
