92 ON THE IMPURITIES OF COMMERCIAL ZINC. 
ing * that the sulphuric acid and the zinc of commerce sometimes contain arsenic, 
nevertheless implies T that he has never obtained decided arsenical spots on porcelain 
. from a commercial sulphuric acid, and subsequently makes this explicit declaration : 
“T declare that I have made this experiment (the test for arsenic by the production of 
arsenical spots) more than five hundred times with the sulphuric acid and zinc of com- 
merce, bought of various manufacturers of chemical products, and have only found 
arsenic three times." t Our own experiments confirm the truth of this statement; of 
all the specimens of zinc which we have examined, not more than two contained 
enough arsenic to give spots on porcelain, and not a single sample of our sulphuric acid 
would have afforded that reaction for arsenic. So long as the chemist, employed upon 
a poisoning case, sought for arsenical spots alone, the little arsenic which his zinc - 
might have contained could do no harm; it is only when important conclusions are 
to be drawn from more refined experiments, with a more delicate apparatus, that the 
arsenic so often present in zinc and acids becomes a matter of very serious concern. 
Our experiments prove that arsenic is contained, not in all samples of commercial zinc, 
but in a large majority of them; and it will be perceived that we arrive at this result 
without impugning in the slightest the accuracy of the experiments upon which the very 
distinguished chemists whose words we have above quoted based opposite conclusions. 
General Remarks. —Yn all commercial zincs metallic lead is the principal impurity, and 
the only one which deserves quantitative determination, if we exclude those exceptional 
cases in which the zinc is only an incidental product, or in which a very bad ore yields 
a zinc contaminated by some special impurity. Such exceptional zincs have only a 
local interest, for they are produced in very small quantities. The zincs which find 
their way into commerce in large quantities are the Silesian and the Belgian, the 
French, English, and American being used at home. 
Silesian Zinc. — The greater part of the zinc sold under this name is made from 
carbonate and silicate of zinc, and from these ores zinc of great purity can be obtained ; 
but wherever blendiferous ores are worked, or zinc-oxide, obtained as an incidental 
product in some other process, is mixed with the ores used, the resulting zinc may be 
expected to contain arsenic. The zincs which we may call collectively German, have 
been more thoroughly studied than any other of the zincs of commerce, and this must 
be our excuse for having examined so few samples of these spelters. It had been 
established by the analyses of Karsten, Wackenroder, Kerl, and others, that lead was 
their chief impurity, and that cadmium often occurred in them in noticeable quantities. 
We have demonstrated the presence of minute quantities of sulphur and arsenic in 
* Annales d'Hygiène Publique, 1839, X XII. 404. t Ibid., 411. $ Ibid., 424. 
