278 MOREY ART. G 



the entropy change in the reaction passes through zero, hence 

 on the K2O side of the hne, K20-2Si02 - K20-2Si02-H20. 

 But unless the K2O content of the vapor is large, which is 

 improbable, the effect will be small; the area, K2O -28102 - 

 K2O • 2Si02 • H2O - V, instead of being zero, will be a very- 

 small quantity which will have but a shght influence on the 

 above relations; the curves, instead of coinciding, would lie 

 very close to each other. 



28. Equilibrium, KiO-SiOi-^H^O + KiO-SiO^ + Solution + 

 Vapor. All the p-t curves so far discussed have had their 

 end-points inside the component triangle ; all of them have gone 

 from one quintuple point to another. Let us now consider 

 one which goes from a quintuple point to a quadruple point in 

 one of the limiting binary systems, e.g., the curve, K20-Si02 + 

 K2O • Si02 • ^H20 + L + V (curve 106), which goes from quintuple 

 point Qi to the quadruple point, K20-Si02 + K2O • Si02 • ^H20 

 + L + V, in the binary system, H2O-K2O • Si02. Since the 

 phases, V, K2O -8102 -^1120, and K20Si02, lie on a straight 

 line, the area of the triangle, V-K2O • Si02 • IH2O-K2O • Si02, is 

 zero, and the equation of the p-t curve reduces to 



dp _ A,„i (V - v") - Au, (V - v") 

 dt A^viiv' -v") - A,,i {v' - v") ' 



in which the accents (') and (") refer to the solid phases, 

 K2O • Si02 and K2O • Si02 • IH2O, respectively. This is evidently 

 the dissociation-pressure curve of K2O • Si02 • ^H20 ; in harmony 

 with our previous conclusions, the slope of the curve, K2O • Si02 -|- 

 K20-Si02-^H20 + L + V (106), is the same as that of the 

 dissociation-pressure curve of K2O • Si02 • IH2O (10a + 106). 

 At the quintuple point it is evident that both numerator and 

 denominator are negative, dp/dt therefore positive. Also, 

 the denominator being much larger than the numerator, the 

 numerical value of dp/dt is less than unity. As the liquid 

 approaches the side of the component triangle along the bound- 

 ary curve, both the triangles A2VI and Aivi diminish in size in 

 about the same proportion, and the value of dp/dt will not 

 change materially. When the liquid gets on the line, H2O- 



