574 JAMES ASTON 
remaining solution will freeze at this constant temperature; but 
because there is no solubility in the solid A and B will separate. 
For a mixture of composition EH, freezing will occur throughout 
the mass simultaneously, with a separation in the solid state of A .. 
and B. In like manner, for alloy M, freezing will be as explained 
above, except that element B will first separate out, with a gradual 
enrichment of mother metal in A, and a gradual lowering of the 
freezing temperature, until at #, there will again occur the isothermal 
solidification of the remaining solution, with separation of A and B. 
COMPLETE INSOLUBILITY 
TEMPERATURE 
TEMPER ATU PE 
TIME A _ COMPOS/7T/ON = 
FIG. 3 
The point of lowest freezing temperature F, is called the eutectic 
point, and the mixture of this composition, the eutectic mixture, or 
the eutectic. 
Summarizing, we have in this type rigidly selective freezing, with 
alloys eutectiferous throughout the range of composition, and no 
tendency for mutual diffusion. Under the microscope we should 
expect to find a more or less segregated excess metal A or B, depend- 
ing upon whether the initial composition was above or below the 
eutectic ratio; together with an intimate composite structure of A 
and B as the eutectic. 
