48 ON THE ALLOYS OF COPPER AND ZINC. 
and a half per cent, I have still deemed it sufficiently accurate for the work to which 
I have applied it; for it must be remembered that these differences are much 
smaller than the variations in composition which may occur in different parts of 
the same cup of alloy, from partial volatilization of the zinc, or from incomplete 
mixture of the metals of which it is composed. 
I may also remark, that although this assay was not adopted by myself until 
after thorough trial of several other processes which seemed to be applicable to the 
special case with which I was occupied, I now consider it a very valuable method. 
It deserves mention in this connection, that the assay by iron has been entirely 
misunderstood by some writers. Thus, Mitchell* directs that the whole of the iron 
added shall be dissolved in chlorhydrie acid, just as if it were zinc, after the copper 
has been completely precipitated ; the absurdity of which procedure is manifest. 
In concluding the subject, I may observe that American assayers are accustomed 
in practice to use a mixture of nitric and chlorhydric acids as a solvent in the 
first instance of the sample to be assayed, and to expel the nitric acid by evaporating 
twice to dryness with an excess of chlorhydric acid. The copper being then pre- 
cipitated from the hot chlorhydrie solution. Some of them collect the copper upon 
a filter, instead of washing by decantation, and rinse it once or twice with very 
dilute chlorhydric acid before washing with water. 
This process has the advantage, that a larger portion of the impurity of the iron 
is soluble in chlorhydric than in sulphuric acid, hence there is less chance of the 
copper being contaminated by it; basic iron salts are also less liable to form; 
the evaporations, which are conducted over the sand-bath, consume somewhat less 
time than when sulphuric acid is used; and the copper, being precipitated in a 
crystalline state, is easily washed without loss, and is not liable to be attacked 
by the acid solution.f 
On the other hand, in the process by sulphuric acid, — which I have used merely in 
order to remove the lead which is contained in the alloys, — the last traces of copper 
seem to be somewhat more readily precipitated from the solution, and the accident 
of the copper adhering too closely to the iron occurs, perhaps, more rarely. 
Of the special precautions which must be taken when this assay is applied to 
* Manual of Pract. Assaying, (London, Bailliére, 1854,) p. 257. 
T The assay undoubtedly affords more accurate results with chlorhydrie acid than when sulphuric acid is 
used. Mr. Dickinson assures me, that in his hands variations so great as 0.5 per cent are entirely excep- 
tional, the usual difference between any two assays of a — sample of ore not being larger than 
0.2 to 0.3 per cent. 
