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ALLOYS l.\ I'KOCKSS OF COM POU NM)ING. 25 I

reckoned with. J f , for example, lead and zinc were melted
together and allowed to slowly cool the two metals would
be almost completely separated. The lead, being the
heavier, would be found at the bottom of the receptacle.
Such mixtures should therefore be thoroughly stirred and
poured at the lowest temperature possible.
Eutectic Alloys.—A\'ith certain alloys there is, on solidi-
fication, a tendency for one of the constituents to become
concentrated at one point, thus giving a casting, if one w-ere
being made, not perfectly homogeneous. " It is no\v well
established that most of the possible associations of any
two metals have more than one point of solidification, and
do not ' freeze ' as pure water does at a single point."
There is, however, in many series of alloys one particular
association of the metals which is more fusible than the
rest of the alloys of the series. This alloy is called the
eutectic alloy, and it possesses a single point of solidifica-
tion ; that is, wdien the eutectic alloy is cooled it sets sharply
as a wdiole at a given temperature. Many associations of
two metals contain an eutectic alloy and consequently have
two points of solidification.
As a molten mass of alloy cools dow-n it begins to soldify
at a certain point, but the eutectic alloy, on account of its
low solidifying point, remains fluid, and entangled in the
portion which has set, until the temperature falls to the
solidifying point of the eutectic alloy, at wdiich temperature
solidification of the mass is completed. There is, therefore,
in many cases abundant opportunity in the inten^al between
the initial point of solidification of the alloy and that of the
eutectic for the mass to arrange itself in a peculiar way
which frecjuently results in a want of uniformity of the mass.
The exact composition of the eutectic alloy is difficult to
determine. The most recent experiments have show^i that
the composition does not in general correspond wdth simple
atomic proportions of the component metals, and this fact.