ON THE ALLOYS OF COPPER AND ZINC. 48 
which had lost zine by volatilization while cooling; from the sheets into which 
the fluid portion of the alloy was cast, I have taken pieces near the edge, which 
were free from any admixture of fragments of the crust. 
Pieces of the alloy weighing from four to twelve grammes, accordingly as they 
contained a greater or less amount of copper, which had been carefully cleaned by 
filing and brushing, were placed in covered porcelain dishes, and dissolved in strong 
nitric acid on the sand-bath, the usual precautions to avoid loss from spattering 
being observed. To the solution thus obtained, sulphuric acid — which had been 
freed from its impurity of lead by dilution with water and subsequent partial 
concentration — was added, and the mixture evaporated to dryness over the water- 
bath; more sulphuric acid being added, and the solution again evaporated until 
all the nitric acid was expelled, or till no trace of blue crystals remained. I have 
rarely found it necessary to evaporate the solution more than twice in order to 
effect this. 
It is of importance that the nitric acid shall be thoroughly removed, for in 
the presence even of a mere trace of this acid it is exceedingly difficult to precipitate, 
by means of iron, the last portions of copper from its solution. 
After all the nitric acid had been expelled, the mixed sulphates were dissolved 
in hot water, and the sulphate of lead (from impurity in the zinc of the alloy) 
allowed to settle; this was then separated and washed with acidulated water, by 
decantation. 
The solution, which had been collected in a capacious beaker, was now largely 
diluted with water and placed upon a sand-bath, the heat of which was regulated 
so that the solution could attain a temperature nearly equal to boiling, without, 
however, coming into actual ebullition. A sheet of the purest iron obtainable 
was next placed in the solution, leaning against the sides of the beaker so that 
the largest possible amount of its surface should be exposed, in order that the 
iron might be attacked equally at all points by the acid, and that the copper 
should be precipitated in even sheets upon it. 
The solution should be acid enough to insure a slight evolution of hydrogen 
during the precipitation. Care should, however, be taken to guard, in the first 
instance, against the addition of too great an excess of acid, lest the iron be cor- 
roded to an unnecessary or even ruinous extent. It is of course desirable to avoid 
dissolving any more iron than is necessary to insure regularity and completeness 
in the precipitation, for in this case a smaller portion of its impurities will be 
exposed, and its surface will remain smoother. 
