90 ON THE IMPURITIES OF COMMERCIAL ZINC. 
still remained a small amount of chloride of gold, which had escaped reduction by the 
sulphurous acid, and to separate this chloride from the solution of arsenious acid, dis- 
tillation was resorted to. The retort in which the residue from the distillation re- 
. mained was washed out with water acidulated with cAlorhydric acid, and the liquid so 
obtained was added to the original distillate, through which sulphuretted hydrogen was 
then passed. The precipitated sulphide was collected on a filter, dried, and redissolved 
in ammonia ; this ammoniacal solution was then evaporated to dryness, and the residue 
weighed as sulphide of arsenic. As the result of this complicated process, which in- 
volves the use of so many different reagents and vessels, Schauefele obtained a quantity 
of arsenic which varied between two thousandths of one per cent of the weight of 
the original zinc, in that sample which contained the most arsenic, and two five-thou- 
sandths of one per cent in that which contained the least. The weight of the sulphide 
of arsenic, which was the final result of the analysis, in no case exceeded ten milli- 
grammes. ‘There is no certainty that this very small amount of arsenic was not de-* 
rived from the acids used in the process, for Schauefele merely states that his sulphuric 
‘acid, of which he used a very large quantity, was “absolutely free from arsenic,” and 
the other reagents are said to be pure. The methods by which he insured this ab- 
solute purity are not even hinted at, and we therefore have no opportunity of judging 
for ourselves of the sufficiency of the processes employed to eliminate the arsenic from 
these reagents, of which two at least almost invariably contain appreciable amounts of 
that impurity. Quantitative determinations, in which the original weights are kilo- 
grammes, and the final weights milligrammes, are trustworthy only when the processes 
are short and simple, and the reagents employed are proved to be above suspicion. 
The process of M. Schauefele was long and complex, and his reagents, so far from 
being proved to be above suspicion, were in all probability the source of the arsenic 
which he attributed to the zinc. In the valuable paper which we have before cited, 
Karsten distinctly states that the Silesian zinc is free from arsenic, basing this state-. 
ment upon experiments which in their general method closely resembled those of 
Schauefele; he endeavored to decompose a solution of nitrate of silver by a stream of 
hydrogen generated by the zinc under examination, but though his process was analo- 
gous to that of Schauefele, he was led to the diametrically opposite conclusion. Again, 
Schauefele’s second method was essentially the process which has been thoroughly 
studied by Jacquelain, who claims for it a delicacy * equal to that of Marsh's process; 
and yet in Jacquelain's own hands this method detected no arsenic in the specimen 
of zinc which he examined.t We cannot avoid the conclusion, that the determina- 
* Ann. de Ch. et Phys, [3.] IX. 490. | T Ibid., 1843, [3.] VII. 189. 
