44 . ON THE ALLOYS OF COPPER AND ZINC. 
After the introduction of the iron, the beaker is to be covered with a glass 
plate, and left to itself until the copper is entirely precipitated, which occurs at the 
end of an hour or two at farthest. Having proved that no more copper is con- 
tained in the solution by testing a small portion of it with sulphuretted-hydrogen 
water, the beaker is removed from the sand-bath, and the clear acid solution 
decanted off as completely as possible from the copper and iron. 
If this solution be diluted with a large quantity of hot water, it immediately 
becomes yellow, then turbid, and in a few minutes deposits a bulky flocculent 
precipitate of a basic salt of peroxide of iron, although the supernatant liquid 
is strongly acid. Since a quantity of this solution remains adhering to the 
metals, it is well, in order to avoid the precipitate which would form if they 
were treated at once with hot water, first to pour a small quantity of cold water 
upon them. The piece of iron is then to be taken from the beaker, and any 
particles of copper which may be loosely attached to it removed by means of the 
wash-bottle; the iron being carefully preserved for subsequent examination. The 
copper, having been once more rinsed with cold water, is now to be treated with 
boiling water, which should be decanted off into a large clean beaker, without 
waiting for all the copper to be deposited, as soon as the liquor assumes a yellow 
color. After two or three additions of hot water, the subsalt of iron is no longer 
precipitated, and the copper may be allowed to subside at its leisure. Any par- 
ticles of copper which are found adhering to the sheet of iron which was used for 
the precipitation must now be removed by gently rubbing them with the finger, 
care being taken not to disturb the black coat* which is found adhering to the 
iron, any more than can be avoided; they are then to be washed into a small 
porcelain capsule, into which is also brought the mixed deposit of subsalt of 
iron and particles of copper collected during the first rapid washing of the copper. 
* The black crust which forms on the iron beneath the layer of copper contains a small amount of 
carbon and some silica, but consists for the most part of what appears to be a compound of silicic acid 
and oxide of iron; this substance, called Slag by Morfit and Booth in their very able * Report upon the 
Chemical Analysis of Cast-Iron Gun-Metal" (vid. Report of Experiments on Strength, &c. of Metals 
for Cannon, by Officers of the Ordnance Department, U. S. Army, (Philadelphia, Baird, 1856, 4to,) ` 
p. 408,) is but sparingly soluble in dilute chlorhydrie acid; slowly soluble in cold, more rapidly in hot 
concentrated chlorhydrie acid; easily soluble in concentrated nitric acid, especially when hot; with sep- 
aration of silica in both cases. It is slowly decomposed by strong soda lye. This “slag” is formed in 
considerably larger quantity when sulphuric acid is used than when chlorhydric acid is substituted for it 
in the assay. As it usually adheres, however, to the iron with considerable firmness, the copper can 
generally be rubbed off without disturbing it materially. : 
