The Ternary System MgO-Al % O x -SW v 313 



(point 6, fig. 1), at a temperature beyond the reach of the 

 platinum resistance furnace, is probably a eutectic ; and 

 a third (point 5, fig. 1) at the end of a flat boundary 

 curve where it has been found impossible to be certain of 

 equilibrium within a reasonable time, may be a eutectic. 

 Point 6 has been located and characterized as a eutectic 

 according to theoretical reasoning based on such experi- 

 mental evidence as could be obtained. If this point is a 

 eutectic its composition must necessarily lie within the 

 triangle MgO, MgO.Al 2 3 , 2MgO.Si0 2 , but if it is not a 

 eutectic its composition must lie outside of this triangle. 

 In either case, however, it is necessary that the three 

 boundary curves which meet at this point be of such a 

 nature that each one will intersect the angle formed by 

 the other two. From these considerations and a glance 

 at points 4, A, and G (fig. 1), the starting points of these 

 three boundaries, 4-6, A-6 and G-6 respectively, it is evi- 

 dent that if point 6 is a eutectic it will probably be of 

 a composition near the line joining MgO.Al 2 3 and 

 2MgO.Si0 2 and inside the triangle MgO.Al 2 3 - MgO - 

 2MgO.Si0 2 , since the general direction of boundary 4-6 

 is fixed by actual data, and theoretically boundary G-6 

 will only intersect the angle formed by A-6 and 4-6 if 

 such is the case. Granting for the moment that point 6 

 is a eutectic, let us consider the nature of indirect exper- 

 imental evidence which would prove this to be so. If 

 point 6 is thus a eutectic the composition line MgO.Al,0 3 - 

 2MgO.Si0 2 will be the projection of the concentration- 

 temperature diagram for the binary eutectic system of 

 these two compounds, and the binary eutectic composition 

 will be at the intersection of this line and the bound- 

 ary 4-6. This point of intersection represents a composi- 

 tion which is the maximum melting temperature on the 

 boundary 4-6. That MgO.Al 2 3 - 2MgO.Si0 2 is a binary 

 eutectic system was shown experimentally by microscop- 

 ical examination of mixtures of these two compounds 

 which had previously been fused in the iridium furnace. 

 These examinations showed such material to contain only 

 these two compounds, while' if MgO.Al 2 3 - 2MgO.Si0 2 

 did not form a true eutectic series, well-developed crys- 

 tals of MgO undoubtedly would have been present. Also 

 such melting temperatures as could be determined for 

 these mixtures in the iridium furnace would tend to show 



