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78 ON THE IMPURITIES OF COMMERCIAL ZINC. 
very probable that the sulphuric acid with which we first experimented contained 
arsenic, inasmuch as we had obtained a distinct deposit of arsenic from that acid and 
the Vieille Montagne zinc, which subsequent experiment had proved to be free from 
that impurity. In order satisfactorily to establish these conclusions, it was necessary 
to prove by frequent repetition that the same result might always be expected from 
these two zincs, and that their freedom from arsenic was a property shared by the 
whole sample, and not an accidental peculiarity of a particular fragment. At sundry 
times we therefore repeated again and again the long and careful test for arsenic above 
described with these two samples of spelter, and invariably arrived at the same con- 
clusion; namely, that no deposit of any kind could be obtained in the reduction- 
. tube from these zincs and purified sulphuric acid. 
Delicacy of the Test. — To prove the sufficiency of our apparatus, and the absence of 
every substance which might be supposed to prevent the formation of the arsenic 
mirror, and, moreover, to obtain mirrors from known quantities of arsenic with which 
roughly to compare deposits obtained in experiments in which the arsenic was the un- 
known quantity, we made the following experiments: — 
1. 200 grammes of Vieille Montagne zinc, about 200 c. c. of purified dilute sulphuric 
acid, and 20 drops of pure chlorhydric acid, were first thoroughly tested for arsenic, 
and found to be perfectly pure. 
2. 200 grammes of Vieille Montagne zinc, and about 200 c. c. of purified sulphuric 
acid, were thoroughly tested, and gave no deposit in the reduction-tube. Into the 
flask whose contents had been thus proved, two tenths of a milligramme of arsenious 
acid (weighed on a Plattner's assay balance, made by Lingke of Freiberg), dissolved in 
20 drops of the same chlorhydric acid used in the first experiment, were introduced. 
An enormous mirror of arsenic appeared instantly in the reduction-tube. To get the 
greatest effect, the arsenious acid should be thoroughly dissolved, and its solution should 
be effected without the use of any but a very gentle heat. 
3. Using the same zinc and the same acids, in the same quantities which were em- 
ployed in the foregoing experiments, and proving the materials as in the last experi- 
ment, we obtained, in half an hour, a very large and distinct mirror of arsenic, by in- 
troducing into the flask one tenth of a milligramme of arsenious acid. 
. 4. 200 grammes of Pennsylvanian zinc and about 200 c. c. of purified sulphuric acids 
were tested for one hour, and proved to be perfectly pure. One milligramme of arseni- 
ous acid had been dissolved in 20 drops of the pure chlorhydric acid which had been 
used in the first experiment, and the solution diluted with distilled water to the bulk 
of 50 c. c. One cubic centimetre of this solution was introduced into the flask whose 
