Alumina with Silica, Lime and Magnesia. 307 



change of direction, but not a minimum. There is, however, 

 an inversion temperature along the line L, where both CaO 

 and 3CaO.Al 2 3 separate at the temperature 1531°. For con- 

 centrations between L and B the lime which has crystal- 

 lized combines to form 3Ca().Al 2 3 . Along the liquidus 

 BC, 3CaO.Al 2 3 is the stable phase. In practice, the time 

 factor is a very important one in bringing about these reactions 

 and some lime always becomes surrounded by crystals of the 

 compound, so that the next phase, 5Ca0.3Al 2 3 , also occurs. 

 This renders the entectic observations along NC more irregular 

 than they would otherwise be and, as the diagram shows, the 

 eutectic, C, occurs as far over as M. 



In order to prepare the pure compound 3CaO.Al 2 3 , it is 

 necessary to bake the charge a long time at about 1400°. This 

 allows diffusion to occur with the elimination of the excess of 

 CaO and 3Ca0.5Al 2 3 Experimentally, we found that 

 the 37*78 per cent charge, held 21 days at 1400°, was free of 

 the excess phases. Similarly, the compositions 35 per cent, 

 34 per cent and 32 per cent of A1 2 3 , when merely fused and 

 crystallized, without the long exposure, show 3CaO.Al 2 3 

 with CaO and 5Ca0.3Al 2 3 , but were transformed into CaO 

 and 3CaO.Al 2 3 by heating at 1400° for the same length of 

 time. In order to accelerate the reaction, we took the pre- 

 viously fused charges and ground them to a fine powder before 

 starting the heat treatment. Such cases as this are not 

 uncommon in silicate melts and the investigator must bear 

 them constantly in mind or he will be led far astray. 



The compound 3CaO.Al 2 3 is isotropic, with density 3*038 

 for the annealed material. It is readily attacked by water, 

 either hot or cold, and dissolves quickly in dilute hydrochloric 

 acid. 



The eutectic (C) between 3CaO.Al 2 3 and 5Ca0.3Al 2 3 at 

 52*22 per cent occurs at 51 per cent A1 2 3 and melts at a 

 temperature of 1382°. 



The eutectic (E) between 5Ca0.3Al 2 3 and CaO.Al 2 3 

 occurs at about 53 per cent A1 2 3 and 1382° C. 



D is the maximum corresponding to the compound 5CaO. 

 3A1 2 3 at 52*22 per cent A1 2 3 , which melts at 1386° C. The 

 maximum is so close to the eutectics, both in composition and 

 temperature, that a precise determination is difficult. It is 

 isotropic, has an index of refraction of about 1*61, and a 

 density of 2*828. 



This compound also occurs in an unstable form which is 

 birefracting, but always changes into the isotropic form when 

 given an opportunity to do so. 



Between 40 per cent and 60 per cent A1 2 3 a number of 

 very small irregular heat effects were observed. It was 



