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Proceedings of the Royal Society of Edinburgh. [Sess. 
alloy, and has a correspondingly fine structure. But during the period of 
solidification of a binary eutectic in a ternary alloy the temperature is not 
constant, and a portion of the mass is always liquid. The particles of the 
binary eutectic are therefore free to migrate, unless the rate of cooling is 
very rapid, and, instead of the fine structure usually associated with 
eutectics, the crystals of each of the two phases are able to grow to a con- 
siderable size, and have exactly the same appearance as primary crystals. 
Fig. 2 shows a lead-tin-bismuth alloy, differing somewhat in composition 
from Charpy’s mixture, but having the same structural constituents. Here 
the large crystals of bismuth at the foot of the micrograph are the primary 
growth, and above them are to be seen smaller but well-formed crystals of 
bismuth and tin, closely intermingled ; the small crystals have been formed 
during the second period of solidification, and represent the binary eutectic. 
At certain places near the foot can be seen dark skeletons of tin adjacent 
to large primary crystals of bismuth which must have absorbed the 
secondary growth of bismuth as it formed. The ground mass consists of 
the ternary eutectic, though its structure is not clearly apparent with 
such a low magnification. 
Other ternary alloys, of more complex character than the above, show 
a corresponding arrangement of the constituents. Thus, in the lead- 
antimony-copper alloys, rich in lead, copper combines with antimony to 
form the purple compound SbCu 2 , and there is a binary eutectic of antimony 
and SbCu 2 . In an alloy containing, say, 66 per cent, of lead, 24 per cent, 
of antimony, and 10 per cent, of copper, the compound SbCu 2 is the primary 
solid, and it is followed by the Sb-SbCu 2 eutectic. A micrograph shows 
that both antimony and SbCu 2 assume primary forms, most of the white 
crystals of the former being traversed by the purple skeletons of the latter 
around which they have grown. Similarly in the tin-antimony-copper 
alloy known as Babbit’s metal, there are apparently two primary substances 
— a skeleton network of the e-solution of copper-tin and compact crystals 
of antimony. 
Thus the form of the crystals in an alloy is determined largely by the 
physical condition of the mixture during the period of solidification ; and 
since, in general, during the whole period of formation of a binary eutectic 
in a ternary alloy a portion of the mass remains liquid, the two constituents 
of this eutectic are able to assume the form of primary crystals. 
