ON THE IMPURITIES OF COMMERCIAL ZINC. 91 
tions given in M. Schauefele's thesis have no general significance, and have gained 
more credit than they deserve. Our observations conclusively prove that there are 
zincs in commerce which are not contaminated with arsenic, and it should be noticed . 
that, while one of our pure samples (the Pennsylvanian) was of a zinc which is not 
yet manufactured in large quantities, the other was a specimen of the Belgian zinc, 
one of the most common and abundant of the commercial spelters. 
We turn now to the discussion of the opposite error, namely, that arsenic is very 
rarely to be found in the zinc of commerce. On this point we need only quote the 
strong statements of the highest authorities. Regnault, in the Report * to the French 
Academy on Marsh's process and its modifications, wrote: ** It is easy to procure in com- 
merce zinc and sulphuric acid which give no arsenic in Marsh's apparatus.” With the 
proper understanding of what is here meant by “ Marsh's apparatus,” this statement 
is as true now as it was twenty years ago. The committee relied chiefly upon the pro- 
duction of arsenical spots on porcelain, and though they recommended a form of ap- 
paratus adapted for heating the arseniuretted hydrogen to redness, yet in this apparatus 
the reduction-tube was not drawn down to a fine bore beyond the heated portion of the 
tube, and the committee in their own experiments seem to have preferred the arsenical 
spots as affording the best evidence of the presence of arsenic. They completely dis- 
solved 500 grammes of commercial zinc in dilute sulphuric acid, and obtained from the 
hydrogen evolved no sensible arsenical spot; the black residue they did not examine. 
The test to which we have submitted our acids and zincs is more delicate than that 
applied by the committee of the French Academy; Otto's apparatus is more sensitive 
than that used by this committee, and will detect the presence of arsenic in quantities 
too small to produce sensible spots. It is self-evident that the continuous deposition 
of arsenie from a stream of hydrogen as it flows steadily through a very fine tube for 
an hour or more, would exhibit an amount of arsenic too minute to give the slightest 
perceptible spot in the instant during which the porcelain surface is held in the e 
ing jet of gas. The first reaction is prolonged and accumulative, the second is inter- 
mittent and instantaneous. Blancard,t in commenting upon the statement regarding 
the ease of obtaining pure zinc, which is above quoted from the Report, has remarked 
with truth, that many zincs of commerce, which give no spots by Marsh's apparatus, 
nevertheless contain sometimes antimony, sometimes arsenic, sometimes both. 
The same explanation should accompany the statements of Orfila, with regard to 
arsenic in commercial zinc and acids. This distinguished toxicologist, in a very valua- 
ble paper on * The Means of being assured of the Presence of Arsenic," after remark- 
* Comptes Rendus, 1841, XII. 1076, and Ann. de Ch. et de Phys, [3.] II. 159. 
t Dingler's Polyt. Jour., 1841, LXX XII. 425, from Jour. de Pharmacie, Sept. 1841, p. 543. 
