80 ON THE IMPURITIES OF COMMERCIAL ZINC. 
4 
opinion, though Marsh, in his original memoir, distinctly says, that “ the presence of 
arsenic in artificial orpiment and realgar, . . . . . and in sulphuret of antimony, may 
be readily shown by this process, when not more than half a grain of any of those com- 
pounds is employed." * When the amounts of arsenic and of sulphuretted hydrogen 
' are alike minute, it is quite certain that the reaction for arsenic is not affected by the 
unavoidable presence of this gas. 
Arsenic in American acids. — We bavé tested four different kinds of American WE 
phuric acid, of which two were commercial oil of vitriol, and two were sold as chemi- 
cally pure acids. The test applied to these acids was always the same, and may be 
described once for all. 100 grammes of Pennsylvanian zinc, from the same bar which 
in many trials had been shown to be free from arsenic, was placed in the flask, and the 
acid to be tested was used instead of the purified acid which was always employed 
when the zinc was the suspected substance. In every experiment, the gas was passed 
through the reduction-tube at least one hour. In experiments several times repeated, 
the sulphuric acid made at Providence, Rhode Island, invariably yielded a distinct 
deposit in the narrow part of the tube. It should be stated, however, that the Provi- 
dence acid used in these repeated experiments all came from one carboy. In a sample 
of the acid manufactured at Waltham, Massachusetts, we detected a similar trace of 
arsenic. 'The deposits obtained from these two acids were hardly larger than that pro- 
duced by the 0.000015 gram. of arsenic used in the fourth experiment on the delicacy - 
of the reaction, but, on the other hand, only a small quantity Jes 25 to 50 c. c.) of the 
acid could be employed in a single experiment. 
The arsenic which is eliminated from these acids during the process of purification 
with chlorhydric acid may easily be collected, and exhibited by Marsh's test. For this 
purpose, the sulphuric acid should be heated with the chlorhydric acid in a flask or 
retort, from which the gas generated is conducted into a small quantity of distilled 
water, kept constantly cool The volatile chloride of arsenic condenses in the water, 
and the arsenic in the solution is readily manifested in Otto's apparatus. The chlor- 
hydric acid used in this experiment must be absolutely free from arsenic; such acid may 
be obtained by passing sulphuretted hydrogen through chlorhydric acid prepared from 
‘salt and pure sulphuric acid. Moreover, the sulphuric acid in the flask or retort must 
be kept fuming during at least half an hour, in order to secure the complete volatili- 
zation of the chloride of arsenic. r 
The chemically pure sulphuric acid, so called, manufactured by Rosengarten, of 
* Edinburgh New Phil. Jour., XXXV. 235. 
