ON THE ALLOYS OF COPPER AND ZINC. 47 
the formation of water in any case in Mr. Dickinson’s experiment with the arsenic- 
tube is an insuperable objection to this explanation, as well as to the idea that 
mixed copper and oxide of copper have reacted upon each other. 
I have determined the quantity of this impurity in some fifty or sixty instances: 
it was found to vary from 1.5 per cent or less to 4 or even 5 per cent, in excep- 
tional cases, of the copper dried at 100°; ordinarily it was from 2 to 2.5 per cent. 
These remarks refer to copper precipitated from sulphuric acid solutions, which is 
usually in a porous spongy state. When precipitated from chlorhydric acid solutions, 
it is usually crystalline and compact, and doubtless contains less impurity. It is said, 
however, that a certain amount of loss will invariably occur, even when the best 
samples of copper precipitated by iron are heated in an atmosphere of hydrogen. 
Even in copper precipitated by pure zinc I have observed that an appreciable loss 
occurs, — varying in my experiments from 0.5 to 2 per cent of the dry copper, — 
when it is ignited in hydrogen gas. It would appear that, while copper may be 
determined with great accuracy when precipitated by pure zinc in a platinum vessel, 
as recommended by Fresenius,* even when the precipitate is dried only at 100^, for 
in this case the copper is deposited in a condition so compact that little or no im- 
purity can adhere to or oxide form upon it, yet it can hardly be determined with 
nicety when precipitated in the spongy state, unless it be ignited in a current of 
hydrogen, or converted into oxide before weighing; it is necessary also to exercise 
the greatest care that none of the smaller particles of copper, are lost during the 
washing, for many samples of copper, when precipitated as sponge by zinc, are 
peculiarly liable to this accident. 
' In regard to the assay which I have attempted to describe, I would remark that 
I have used iron instead of zinc,t simply because I was, at the time, unable to 
procure a sufficient quantity of the latter in a state pure enough for analytical 
purposes. The assay by iron, as I have used it, is clearly not an accurate analytical 
process, but, on the other hand, it is in several respects preferable — as an assay — 
to the method by zinc, and will be found, after a little practice, to yield satisfactory 
results. It is doubtless as good a method as any which we possess, wherever 
absolute accuracy is not required, and where a large number of determinations must 
be made. Admitting that the results obtained by it may vary among themselves 
within the limits of one per cent, or in exceptional cases even to the extent of one 
* Anleitung zur Quantitative chemischen Analyse, (Braunschweig, 1858,) S. 247. 
f Method proposed by Vauquelin, Annales de Chimie, (1798,) X XVIII. 50. 
