96 Ferguson and Merwin — The Ternary System. 



The limits of the lime field were obtained by interpola- 

 tion. One quench at 1660 °C of a charge with a composi- 

 tion lime 55, magnesia 25, silica 20, showed no glass and 

 in it crystals of 2CaO.Si0 2 and lime could be identified. 

 If an equilibrium condition had been reached this indi- 

 cates a eutectic relation between lime, magnesia and 

 a2CaO.Si0 2 and this we have assumed to be true in 

 making our diagrams. However, Rankin and Wright 

 found that, in the binary system, lime and silica first gave 

 rise to 2CaO.Si0 2 and lime before combining to give 

 3CaO.Si0 2 , and a similar condition may have been 

 encountered by us. If a eutectic exists (as we have 

 assumed to be the case) the temperature of it must 

 lie above 1900° C, the decomposition temperature of 

 3CaO.Si0 2 . 



The heat treatment of charges in which the compound 

 2CaO.Mg0.2Si0 2 occurs as a primary or a secondary 

 phase, as given in Table I, may seem to be far too short 

 a time in view of the tendency shown by solutions of this 

 compound to undercool. Such is, however, not the case, 

 since if care be taken to start with fully crystallized 

 material, and the temperature of the charge be never 

 allowed to exceed the desired temperature, this difficulty 

 can be and was avoided. In studies of this nature the 

 length of the heat treatment of itself means but little 

 unless the properties of the reacting phases are known 

 and these may be of such a character as to necessitate 

 a knowledge of the original state and previous history 

 of each charge before one can judge if the heat treatment 

 has been sufficient. The experiments carried out at 

 temperatures above 1600 °C were made in the cascade 

 furnace 38 designed for the determination of the melting 

 point of cristobalite. 



Temperature Relations along the Boundary Lines exclusive 

 of Quintuple Points. 



The temperatures at which the complete fusion of the 

 charges with compositions represented by the boundary 

 lines takes place, may be determined either directly upon 

 such charges, or indirectly by following the crystalliza- 

 tion curves of the compositions which lie within the 

 adjacent fields. This latter method offers no especial 



33 J. B. Ferguson and H. E. Merwin, this Journal, 46, 417, 1918. 



