ON THE IMPURITIES OF COMMERCIAL ZINC. 89 
there seems to be little reason for expecting the complete removal of the arsenic, inas- 
much as the fused saltpetre can only be brought in contact-with the external surface 
of the zinc, however finely the metal may be granulated. It is not inconceivable that 
a trace of arsenic in a zinc should be eliminated by Meillet's process, and that a sam- 
ple, priginally almost absolutely free from arsenic, should be so improved as to afford 
no perceptible mirror; thus Stein * could not detect arsenic in a sample of zinc purified 
by this method, but as a general rule it will not be safe to rely upon this process for 
the conversion of arsenical commercial zinc into zinc fit for use in Otto's apparatus. 
On this subject of arsenic in commercial zinc two opposite errors demand notice. 
On the one hand, not a few chemists have maintained that commercial zinc almost 
invariably contains arsenic, and that Marsh's test is untrustworthy on this account. 
Thus Persoz T states that the greater part of the zinc sold in Strasbourg contains 
arsenic, and in a previous paper, condemning Marsh’s process, he remarks with truth, 
that even distilled zinc may give spots of arsenic. The opinion that all zinc contains 
arsenic, finds support in the quantitative determinations by Schauefele of the amount 
of arsenic in French zinc, Silesian zinc, and Vieille Montagne zinc respectively. These” 
determinations have been quoted in almost all the modern text-books, and have had in 
our opinion much more weight than they are really entitled to. Schauefele determined 
the arsenic in his samples of zinc by two methods. The first was that of Villain, and 
consisted in counting the number of arsenic spots obtained from a given weight of 
zinc, and estimating the corresponding amount of arsenic by means of the following 
absurd rule: — one milligramme of arsenious acid will give two hundred and twenty- 
six arsenical spots two millimetres in diameter. The utter unfitness of this process for 
exact experiments is too obvious to need any illustration. In applying this singular 
: method, Schauefele completely dissolved one kilogramme of zinc in dilute sulphuric acid, 
but in this connection says not a word about the purity of the acid, of which he must 
have used at least one kilogramme and a half. The second method used by Schauefele 
was essentially that described by Jacquelain, and consisted in passing all the hydrogen 
generated by a given weight (from 320 to 800 grammes) of zinc through a solution of 
chloride of gold; this solution was partially decomposed by the arseniuretted [sulphu- 
retted ?] hydrogen, and when the zinc had been completely dissolved. the chloride of 
gold solution, which was supposed to contain all the arsenic of the zinc in the condi- 
tion of arsenious acid, was further decomposed as completely as possible by means of 
sulphurous acid, and the precipitated gold separated by filtration. In the filtrate there 
* Jour. f. pr. Chem., 1851, LIII. 40. 
f Ann. de Ch. et Sait, [3.] 1844, X. 507, note. 
12 
1 Ibid., [2.] 1840, LXXIV. 432. 
VOL. VIII. 
