258 MOREY ART. a 



system, liquid-solid. It should be pointed out that this mini- 

 mum melting point is not at the point of maximum tempera- 

 ture, but at a lower temperature. The point of maximum 

 temperature is found at such a salt content that the denominator 

 becomes zero, as previously stated, while the minimum melting 

 point lies at a slightly higher salt content, and a lower tempera- 

 ture and pressure. In a system containing a volatile component 

 the point of maximum temperature is not at the composition of 

 the compound, as is the case in systems of non-volatile com- 

 ponents or in condensed systems, but at a composition slightly 

 displaced toward the volatile component. In the case of 

 CaCl2 -61120 the difference is very small, and the two points 

 have never been separated, but at higher temperatures and 

 pressures the difference is no longer negligible. 



After the minimum melting point has been passed, the coeffi- 

 cient of the second term in the denominator becomes negative, 

 so that in both numerator and denominator the second term, 

 the entropy and volume differences (solid-liquid), in themselves 

 negative, are multiplied by a negative coefficient, hence the 

 second term in both becomes positive, and is to be added to the 

 positive first terms. The slope of the p-t curve is then posi- 

 tive, and remains so until the invariant point, CaCl2 -61120 

 -f CaCl2 - 4H2O + solution + vapor, is reached, at which a new 

 solid phase, calcium chloride tetrahydrate, makes its appearance. 

 The p-t curves that proceed from this invariant point when dif- 

 ferent phases disappear present some novel features, and are 

 considered in detail below. 



18. Correlation of the t-x and p-t Curves. The sequence 

 of the points of maximum temperature and minimum melting 

 point on the three-phase curve, vapor + liquid (saturated solu- 

 tion) + CaCl2-6H20, is brought out especially well by the appli- 

 cation of equation (14), which in this case becomes 



d^ 1 dfxi Av'' (0 - x^) - Av'-^ (0.5066 - a:0 



dx^ ~ 1 — x'- dx^ Av'^ At;"' — Ay"' Atj*' 



As before, the denominator is positive, and the sign of the 

 numerator is determined by the sign of (x* — x^) = (0.5066 — x^). 

 When the difference (x' — x^) is large and positive, the 



