ON THE IMPURITIES OF COMMERCIAL ZINC. 87 
Lastly, we submitted our four specimens of English zinc to the test for arsenic. 
With purified acid, 200 grammes of the zinc made by Dillwyn and Company began to 
show a deposit in the reduction-tube within ten minutes of the commencement of the 
experiment, and in twenty-five minutes this deposit increased to a very perceptible 
mirror. A similar result was obtained from 200 grammes of the Mines Royal zinc. In 
fifteen minutes the stream of hydrogen from this zinc began to deposit arsenic in the 
reduction-tube, and at the end of three quarters of an hour, a thin but perfectly dis- 
tinct mirror extended over three or four centimetres of the fine tube. On the label 
which accompanied the specimen of the spelter made at Minera, near Wrexham, it 
was stated that the zinc was manufactured from silicate of zinc, and we therefore 
expected to find this spelter purer than the ordinary English zinc made from blende; 
but, on the contrary, it contained a large amount of lead, and the test for arsenic showed 
it to contain more of that impurity than either of the two specimens before examined. 
100 grammes of the Wrexham zinc began to show a deposit in the reduction-tube in ten 
minutes from the beginning of the experiment, and at the end of an hour a mirror 
had accumulated large enough to be identified by the arsenical odor. But of the four 
samples of English zinc, that of the Messrs. Vivian contained the most arsenic; 200 
grammes of this spelter yielded an enormous mirror of arsenic in less than ten minutes, 
and in a few minutes more a second mirror, large enough to give the characteristic 
odor. With regard to English zinc, therefore, our observations do not agree with 
those of Brett,* who states that he has examined many specimens of English and 
foreign zincs, and could never obtain any indications of arsenic when the sulphuric 
acid was pure. The explanation of this discrepancy is to be found in the fact, that 
the test applied by Brett, although essentially Marsh's test, had not the extraordinary 
. delicacy which is insured by Otto's form of Marsh's apparatus. 
This question now suggested itself: In presence of an excess of zinc, is not arsenic 
retained in the black residue (lead) from zinc dissolved in dilute acids, in such a con- 
dition, that it is not attacked by the acids or by the hydrogen, and therefore escapes 
detection? To determine this point, if possible, we dissolved 40 grammes of the Vieille 
Montagne zinc, which had showed no trace of arsenic by Marsh's test in dilute sul- 
phuric acid, free from arsenic. The residue obtained was washed, treated with chlor- 
hydric and a few drops of nitric acid, and the solution gently evaporated to*a small 
bulk in presence of a little pure sulphuric acid. The mixture thus obtained was 
- washed into the flask of Otto's apparatus, whose previous contents of zinc and acid had 
. 
* Philosophical Magazine, 1842, [3.] XX. 404. 
