THE CONSTITUTION OF THE COPPER-TIN SERIES OF ALLOYS. 29 
almost free from cores, but some weeks after etching, large faint cores were found to 
have developed, so that the « is not absolutely uniform. 
However, it appears evident that, with a very little allowance for lag in tlie 
transformations due to too rapid a cooling, we are entitled to say that from a chemical 
point of view the alloy Sn 2 solidifies to a completely uniform solid at a temperature 
near 900° The resulting solid must therefore be a solid solution, or, what is the same 
thing, an isomorphous crystalline mixture of copper and tin, or of copper and some 
comjDound of the two metals. The process of solidincation levealed hy these chdls, 
in winch the solid phase differs considerably in tin content from the licpiid phase, liut 
gradually absorbs it, is an exact verification of Koozeboom’s theory ; we must 
infer that the a combs seen at the various stages of the solidification are all solid 
solutions. 
The absence of cores in the higher chills must he interpreted as indicating that the 
adjustment between the successive solid and liquid phases was, at these high 
temperatures, a fairly perfect one, but it is not improbable that, with still slower 
cooling, the mother-substance would have disappeared at a somewhat higher tem¬ 
perature. It is obvious that while each a crystal is bathed on all sides by mother- 
substance, it will be favourably situated for maintaining its uniformity by diffusion 
from the margin to the centre ; hut when the solidification has so far progressed 
that the crystals touch each other over a great part of their suiTace, the remaining 
drops of mother-liquid will he very liable to react with the margins only of the large 
combs, and to leave the cores of the crystals unduly rich in copper. This retardation 
will also have as a result that the complete solidification of the alloy wdl occur at an 
unduly low temperature. ^Ve shall find examples of this later on in spite of all our 
efforts to attain perfect equilibrium by slow cooling. 
Sn 2. V.8.C. chill at 800° (not reproduced). 
This is a beautifully uniform alloy. There is no mother-substance. The polished 
and etched surface contains two patches belonging to two crystals, distinguished hy 
reflecting light differently. These two touch along a continuous line without a trace 
of mother-substance between them. We have here a very good proof of the solid 
solution theory, for otherwise, what has become of the tin-rich matter ? However, 
when freshly etched with HCl, a lens shows large cores differing very little from the 
rest of the matter round them, so that the process referred to above has occurred. 
These cores are lost under the microscope. 
Sn 2. V.s.c. chill at 770° (fig. 5). 
This consists of two or three homogeneous grains of a, fitting closely together 
without mother-substance. There is a trace of vague coreing."^ We do not reproduce 
* The striations visible in the lighter grains of a in fig. 5 are due to scratches, which are almost 
inevitable in polishing this very soft material. 
