ON THE ALLOYS OF COPPER AND ZINC. 29 
has enabled one to judge when the crust is ready to be pierced — to obtain perfect 
cups lined with well-formed crystals. 
The chief difficulty which presents itself depends upon the fact that the alloys 
containing more than 80 to 85 per cent of copper solidify much more rapidly on 
top than upon the sides or at the bottom of the crucible. It is therefore necessary 
to allow a very thick crust to form before piercing, or no cup at all will have been 
formed, and the alloy will flow out entirely in the fluid state, with the exception 
of the upper crust. 
. Fine cups are formed by all the alloys between 80 and 45 per cent of copper, 
the largest crystals being obtained when the crust is pierced while still quite thin; 
while the white alloys containing less than 40 per cent of copper solidify, if anything, 
more rapidly on the sides and at the bottom of the crucible, rendering it necessary 
to pour out what alloy is still fluid almost as soon as a crust begins to form on top. 
All the white alloys are liable to pass suddenly into a pasty plastic state similar to 
that assumed by zinc or by soft-solder while solidifying. On account of this pecu- 
liarity, it is exceedingly difficult to crystallize them. I have succeeded, however, in 
obtaining crystals of alloys as low down as 30 per cent of copper, and have no doubt 
that, by repeated trials, they might be obtained from alloys still richer in zinc. 
These crystals are all octahedral, usually somewhat elongated and apparently 
much modified by the circumstances in which they have been formed. The edges 
of all of them are rounded. The octahedra are in general more largely developed 
upon one side than upon the other, apparently upon the side from which the last 
drippings of the melted metal fell. They are, moreover, combined together with 
parallel axes, which give to the crystals a striated appearance (see Figs. 1l, 2, and 3); 
these strie are not sharply defined, but their edges have the rounded character 
of the edges of the crystals. This general character is maintained throughout 
the whole series of crystals, from those of pure copper down to those of the lowest 
white alloys which I have obtained. No doubt can possibly be entertained of 
the complete resemblance of these crystals to each other throughout the series, while 
the striking similarity to the well-known crystals of pure copper (obtained by fusion) 
which they exhibit, strongly indicates that they belong to the regular system. 
As it is of course impossible to measure the angles of such crystals, they cannot 
be crystallographically determined; but the most obvious conclusion is, that they 
are monometric. This opinion, however, must be based rather upon analogy than 
on any distinct measurements. ` 
Upon the assumption that the crystals which I have described belong to the 
