12 ON THE IMPURITIES OF COMMERCIAL ZINC, 
with which the sulphuric acid is contaminated. Every specimen of zinc in our posses- 
sion develops sulphuretted hydrogen when treated with dilute sulphuric or chlorhydric 
acid, as may be manifested by placing a slip of paper moistened with alkaline acetate 
of lead in the neck of the flask which contains the zinc and acid. But the ques- 
tion recurs, What is the source of the sulphur which is necessary for the generation 
of this gas? Is it contained in the zinc, or is it derived from the acids used in the 
experiment? To obtain a satisfactory solution of this problem, it is necessary to use 
an acid which does not contain sulphur in any form. Sulphuric acid will not answer 
the purposes we have in view in this experiment; for though it is undoubtedly possi- 
ble to prepare sulphuric acid free from sulphurous acid, yet the doubt would still re- 
main concerning the reduction of the sulphuric acid by the hydrogen, — a reduction not 
impossible at certain temperatures and in certain states of concentration. In testing 
for a minute trace of sulphur in zinc, it is evidently undesirable to employ a reagent 
which contains sulphur, in however stable a combination. Sulphuric acid being then ex- 
cluded, will chlorhydric acid answer the purpose? It is easy to prepare chlorhydric acid 
which gives no precipitate with baryta salts, but it is very difficult to prepare this acid 
from common salt and sulphuric acid, so that, while containing no chlorine, it shall be. 
absolutely free from sulphurous acid, or some lower compounds of sulphur. Loewen- 
thal's* test with sesquichloride of iron and ferricyanide of potassium will reveal the pres- 
ence of such compounds of sulphur in chlorhydric acid, made with the utmost care, and: 
in other respects pure. By means of chlorine or some similar oxidizing agent, these com- 
pounds of sulphur may undoubtedly be oxidized, or at least the larger part of these ad- 
mixtures may be converted into sulphuric acid; whether their last traces can be oxidized 
in this way is a point by no means beyond a doubt. But if the chlorhydric acid has been 
treated with chlorine to accomplish this oxidation, it becomes necessary to remove from 
the acid the excess of the oxidizing agent, for sulphuretted hydrogen would not be de- 
veloped from zinc, contaminated with sulphur, by an acid which contained free chlorine, 
or any substance of like properties. Thus common chlorhydric acid very often contains 
free chlorine, and no zinc will yield with such acid anything more than a very uncer- 
tain reaction for sulphur on lead-paper. To obtain chlorhydric acid which was above 
suspicion, and unquestionably free from every trace of sulphur, and from every oxidiz- 
ing agent which might interfere with our reaction for sulphur in zinc, we found so 
difficult a task, that we finally rejected this acid “altogether, and resorted to the follow- 
ing process. A solution of chloride of calcium, free from every trace of sulphur, was 
D 
* Jour. pr. Chem., LX. 267. 
