ON THE IMPURITIES OF COMMERCIAL ZINC. YA 
adduce positive experiments to show that any compound of sulphur with a metal, which 
might be present in the zinc, would, in all probability, be decomposed in presence of an 
excess of zinc and free acid. , 
Wackenroder, in the memoir already cited, distinctly states that.the black residue 
from zinc is sulphide of lead, — a statement-at first sight sufficiently plausible, but 
really inconsistent with ti. facts of the case. When precipitated sulphide of lead is 
mixed with a large excess of granulated zinc (Silesian, Vieille Montagne, or Pennsylva- 
nian), and treated with moderately dilute sulphuric or chlorhydric acid, the black sul- 
phide soon entirely disappears, while torrents of sulphuretted hydrogen are evolved. 
If, after all the zinc has been completely dissolved, the insoluble residue is fused before 
the blowpipe with carbonate of soda free from sulphuric acid, the mass thus obtained 
will not blacken silver. 
If powdered galena be substituted for precipitated sulphide of lead, the same effects 
will be produced, though much more slowly. The sulphide of lead, therefore, suffers 
complete decomposition in presence of an excess of zinc and free acid, and it is of 
course absolutely impossible that this substance should be found in the insoluble 
residue.* 
The presence of sulphur in the insoluble residue from zinc is, without doubt, very rare; 
but it is also an unquestionable fact, that a certain amount of sulphuretted hydrogen 
gas is generated whenever commercial zinc is treated with dilute acids. This phenom- 
enon has been often observed. Thus Blancard f remarks, “that the sulphur often 
contained in commercial zinc may be shown by bringing paper wet with acetate of lead 
in contact with the gas developed therefrom.”  Fordos and Gélis f say, that “ the for- 
mation of this gas (sulphuretted hydrogen) can only be attributed to the partial reduc- | 
tion of the sulphuric acid by the nascent hydrogen.” Subsequently Jacquelain,§ 
doubting this supposed reduction of sulphuric acid, attributes the production of sul- 
phuretted hydrogen to the presence of sulphurous acid or other compounds of sulphur, 
* We have observed that the sulphides of tin and copper are also decomposed when mixed with an excess 
of zinc and dilute acid. The sulphide of copper was rapidly decomposed, and the residue, after all the zinc 
. had been dissolved, yielded only a very uncertain trace of sulphur before the blowpipe. Precipitated bi- 
sulphide of tin was decomposed much less readily, and when all the zinc had disappeared, the residue gave in- 
dications of sulphur before the blowpipe. Although this might have arisen from some impurity in the tinfoil 
from which the sulphide was prepared, yet it was evident that the decomposition of the sulphide of tin is 
effected with much greater difficulty than that of the sulphides of lead and copper. 
+ Jour. de Pharmacie, 1841, p. 543, in Dingler’s Polyt. Jour. 1841, LXXXII. 425. 
{ Comptes Rendus, 1841, XIII. 437. 
$ Ann. de Ch. et Phys., 1843, [3.] VIL 189. 
