ON THE IMPURITIES OF COMMERCIAL ZINC. T9. 
into error by the close external resemblance of the sulphide of arsenic to the sulphide 
of cadmium, which last metal, not recognized till 1817, has since been shown to be a 
very common admixture in the zinc of commerce. The invention of Marsh's apparatus, 
in 1835, gave to chemistry a test for arsenic of most wonderful delicacy; and Marsh * 
himself, in his original memoir, describing his process, remarks, that “the only am- 
biguity that can possibly arise in the mode of operating above described, arises from 
the circumstance that some samples of the zinc of commerce themselves contain 
arsenic.” But Marsh, thus careful to suspect his zinc, says not a word about the 
purity of his acid, and many observers since Marsh have been more ready to attribute 
the infinitesimal trace of arsenic, which his process has enabled them to detect, to the 
zinc, than to the acids they have used. Schauefele + has actually attempted to deter- 
mine quantitatively the per cent of arsenic present in French, Silesian, and Vieille Mon- 
tagne zincs, and his results have been quoted in many recent handbooks and treatises 
on toxicology. 
The conclusions at which we have arrived, after a long course of experiments with 
many different zincs, and various acids, are these: — first, that much of the zinc of 
commerce is free from arsenic, or at least contains no arsenic that can be detected by 
the most delicate tests known for that metal; secondly, that the sulphuric and chlor- 
hydric acids found in commerce do very often contain arsenic, and are always so liable 
to contain it as to be utterly unfit for use in Marsh's process without special purifica- 
tion for that purpose. The steps by which we were led to these results, and the evi- 
dence on which they are founded, we proceed to describe. We have used exclusively 
"Marsh's process for the detection of arsenic, applied with the apparatus and with 
all the precautions recommended by Otto.f Our apparatus consisted of a flask 
provided with a funnel-tube, and a tube bent at right angles, with which were con- 
nected by connectors of sheet India-rubber, first,.a tube of the form of a chloride of 
calcium tube, filled with asbestos; secondly, a similar tube, filled with pumice-stone 
soaked in caustic potassa; and thirdly, one filled with chloride of calcium. Through 
these three tubes, in the order in which they are named, the gas generated in the 
flask was obliged to pass before it arrived at the reduction-tube, which was of hard 
German pus about one centimetre in diameter. The reduction-tube was drawn 
* Edinburgh Now Phil. Jour., XXXV. 235. 
f Extract from a thesis presented by M. Schauefele. Jour. de Chimie Médicale, [3.] VI. 173; also in 
Dingler’s Polyt. Jour., 1850, CXVI. 248. 
i A Manual of the Detection of Poisons. 
Bailliére. 1857. 
Translated from the German by Elderhorst. New York: 
