36 Rankin and Wright — Ternary System CaO-Al^O^-SiO^. 



which two crystalline phases are obtained imbedded in glass 

 (liquid), as observed with the microscope. 



The data obtained for temperatures along the boundary 

 curves will not be presented, as the essential temperatures for 

 these curves are given later as quintuple and quadruple point 

 temperatures. 



The Quintuple Points. 



A quintuple point, a common point for three boundary 

 curves, is the point at which three solid phases are in equili- 

 brium with solution and vapor. A quintuple point is known 

 as a eutectic if its temperature is the lowest temperature on 

 the three boundary curves of which it is a common point. 



The temperature of a quintuple point may be determined in 

 any mixture of the solid phases present at that quintuple point 

 (for all ternary mixtures contain more or less of the quintuple 

 point composition) either from heating curves, or from quenches 

 by which it is possible to determine the temperature above which 

 two crystalline phases are obtained imbedded in glass (liquid) 

 and below which three solid crystalline phases are obtained ; 

 that is, the temperature at which glass first appears in a ter- 

 nary mixture is the temperature of the quintuple point for that 

 mixture. In presenting the data for quintuple points we will 

 first consider the eutectics, of which there are 8, and then the 

 13 quintuple points not eutectics 



Eutectics. — In dealing with alloys, which contain more or 

 less of a eutectic mixture, there is often developed what is 

 known as a " eutectic structure " which is easily recognized by 

 use of the microscope. So far as we have worked with silicate 

 melts phenomena of that sort are rarely observed ; in other 

 words, a eutectic mixture of silicates has in general no special 

 structure different from any other mixture. On the other hand, 

 eutectic mixtures of silicates are extremely fine-grained and it 

 is extremely difficult to develop or grow crystals in a charge 

 containing a large amount of a eutectic. 



Following is a list of the eutectics, giving compositions in 

 weight percentages of CaO, A1 2 3 , Si0 2 , and the corresponding 

 temperatures. All of the data on which is based the tempera- 

 ture of one single eutectic are given, but for lack of space it will 

 be possible to give only one value (the mean of a large. number 

 of determinations) for each of the other eutectics. The num- 

 ber assigned to each eutectic corresponds to the number in 

 fig. 6. 



Point 2 is the eutectic for Si0 2 , aCaSiG 3 and CaO.Al 2 3 .2Si0 2 , 

 (CaO 23-25, AI 2 () 3 14-75, Si0 2 62). Its melting temperature 

 (1170° ± 5°) is based on the values obtained from various com- 

 positions as given in Table XI. 



