16 ON THE IMPURITIES OF COMMERCIAL ZINC. 
down to a fine bore, and during the progress of an experiment was heated by one of 
Bunsen's triple gas-burners. To prevent any elevation of the temperature in the flask 
during an experiment, it was immersed in cold water, and the dilute acid used was 
always cold, and added in small quantities, With this apparatus (which for conven- 
ience we shall designate as Otto's apparatus), taking every possible precaution to insure 
its perfect cleanness, we made several experiments upon Silesian zinc. 200 grammes 
of this spelter, carefully granulated, were used in each experiment, and the sulphuric 
acid employed was a commercial acid made in this country from Sicily sulphur. We 
were not unaware of the fact, that arsenic is almost invariably found in the foreign 
sulphuric acid made from various impure sulphurs of unknown origin, or from 
pyrites;* but it is a common impression that the American acid manufactured directly 
from Sicily sulphur is free from arsenic. Positive statements to this effect have been 
made by chemists who have had mainly in view the common use of sulphuric acid in ` 
the preparation of chemical compounds used in pharmacy, and the assertion has enough 
plausibility to command ready and general belief. Using such acid and 200 grammes of 
granulated Silesian zinc, we obtained, at the end of the hour during which the reduc- 
tion-tube was heated, a deposit of arsenic perfectly distinct, though not bright enough 
to be called a mirror. Our next experiment was made with the same acid upon 200 
grammes of an excellent sample of Vieille Montagne zinc, perfectly clean and carefully 
granulated. At the end of the hour during which the gas was passed through the 
reduction-tube, a brownish, volatile coating was found in that part of the tube where 
the bore was smallest. These experiments on Silesian and Vieille Montagne zincs 
were several times repeated, and always with the same result; the deposit in the reduc- 
tion-tube was often too thin and slight to be positively identified as arsenic, but it 
could not be distinguished from the deposit of that metal, and would have been per- 
fectly fatal in a medico-legal investigation, or in any case in which absolute purity of 
* On the subject of arsenic in foreign sulphuric acid, the following authorities may be referred to: — 
Martius, Schweigger's Jour. f. Ch. u. Phys., 1811, III. 363. 
Wackenroder, Ann. der Pharm., 1834, XII. 189. 
Wackenroder, Ann. der Pharm., 1835, XIII. 241. - 
Vogel, Jour. f. pr. Ch., 1835, IV. 239. 
Ficinus, Ann. der Pharm., 1835, XV. 77. - 
Berzelius, in his Jahres-Bericht, 1841, X X. 192. 
Brett, Philosophieal Mag., 1842, [3.] XX. 404. 
Schnedermann and Wöhler, Jour. f. pr. Ch., 1845, XXXV. 186. 
Dupasquier, Comptes Rendus, 1845, XX. 794. 
Cameron, Chem. Gazette, No. 320, p. 75, in Jour. f. pr. Ch., 1856, LXVIII. 64. 
