kin and Wright — Ternary System CaO-Al 2 O z -Si0 2 . 33 



XIII. The field of tricalcic silicate, 3CaO.SiO, {16-17-18). 

 3CaO.Si0 2 like 3CaO. A1 2 3 is unstable at its melting point, but 

 unlike 3CaO.Al,0 3 the Held of stability for 3CaO.Si0 2 lies 

 wholly within the ternary system. This field is so narrow that 

 the slope of the melting surface was deduced from the temper- 

 atures along the lines which bound it. 



XIV. The field of lime, CaO(D-CaO- C).— Practically all 

 the melting temperatures within this field are too high for 

 determination, and hence the slope of its melting surface was 

 deduced from the melting temperatures of the boundaries. 



The Boundary Curves. 



A boundary curve is the line which separates two fields and 

 represents the temperatures and concentrations at which the 

 solid phase of one field is in equilibrium with the solid phase 

 of the other field in coexistence with solution and vapor. 



The boundary lines in fig. 6 represent the projection upon 

 the horizontal plane of the lines on the concentration-temper- 

 ature solid model. In order to make clear the way in which 

 the temperature varies along these lines, we have made pro- 

 jections on to a vertical plane, obtaining in this way the 

 series of boundary curves reproduced in fig. 7, in which 

 the numbers and letters used correspond to those in fig. 6. 

 The ordinates represent temperatures, which were determined 

 experimentally. 



The direction of rising temperature along the boundary 

 curves (fig. 7) can be predicted by the application of the theo- 

 rem of Alkemade to the projections of these curves (fig. 6). 

 For example, consider the boundary (5-2) in fig. 6, between 

 CaSi0 3 and CaO. Al 2 3 .2Si0 2 . It is quite evident that a straight 

 line connecting the compositions of these two compounds 

 would intersect the boundary (5-2), and from the theorem of 

 Alkemade it follows that the point of intersection is a max- 

 imum temperature on this boundary. The concentration-tem- 

 perature diagram for boundary (5-2) in fig. 7 shows this 

 maximum. There are in fig. 6 nine boundary lines which one 

 would expect to possess maximum temperatures, and the con- 

 centration-temperature diagram (fig. 7) for each of these bound- 

 ary curves is found experimentally to show such a maximum. 

 If now we consider boundary (1-9) between Al 2 Si0 6 and CaO. 

 Al 2 3 .2SiO„ it is evident that a straight line connecting the 

 compositions of these two compounds would intersect bound- 

 ary 1-9 only if this curve were produced beyond point 9. 

 Hence from the theorem of Alkemade it follows that temper- 

 ature along 1-9 should rise continuously from point 1 to point 

 9. The concentration-temperature diagram for boundary 1-9 in 



Am. Jour. Sci.— Fourth Series, Vol. XXXIX, No. 229.— January, 1915. 

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