558 



which has suddenly been cooled from a lemperature above 407° 

 to 0°, differs considerably from that of a mixture that has been 

 allowed to cool down slotvly. In case of rapid cooling the 

 heat of solution appeared namely to be much smaller negative, 

 which points to an exothermal process, which failed to appear in 

 case of rapid cooling. After continued heating at 85° — 150° the 

 negati\'e value of the heat of solution rose, and after a few days it 

 had acquired the value of the mechanical mixture. 



It was on the ground of these phenomena that K. and Z. con- 

 cluded that the isomorphous mixtures are decomposed at 300° — 407°, 

 so that solid solutions are formed, which already at 100" — 150° 

 consist almost exclusively of the components. 



This conclusion is also optically confirmed, for as Ostwald already 

 observed, the suddenly cooled mass gives the impression of white 

 enamel, whereas the salts taken separately yield a perfectly transparent 

 solid substance when the melt is cooled. On slow cooling it is now 

 clearly obser\'able that the enamel lar substance is converted to a 

 transparent mass. 



Though in these experimental data clear indications are to be found 

 for the probable explanation of the observed phenomena, and this 

 explanation is easy to test, Kurnakow and ZkmczuZxNyi have evidently 

 not realized this, for they conclude : "Weitere Untersuchungen mussen 

 zeigen, ob die Zersetsung der feslen Lösungen von Erscheinungen 

 des Polymorphismus oder von anderen Ursachen abhangt." As 

 however these "weitere Untersuchungen" seem not to have been 

 undertaken by K. and Z., and as the question under discussion is 

 an exceedingly important one, which can be answered in a simple 

 way, we undertook the following investigation. 



2 Ostwald's experiments and those of Kurnakow and Zkmczuznyi 

 about the heat of solution show that the stable state of equilibrium 

 at the ordinary temperature is most probably an equilibrium of two 

 mixed crystal phases, one of which consists practically of KCl, and 

 the other of NaCl 



If this be true, we meet here with the interesting case that at 

 higher temperature these coexisting mixed crystal phases approacii 

 each other more and more in concentration, and become perfectly 

 identical at 407°, or in other words they exhibit an upper critical 

 mixing-pomt. 



The top of the line PQR in Fig. 1 lying at about 66.6 mol. 7„ 

 NaCl, the possibility of the occurrence of a compound might, however, 

 also be thought of It should be noted, however, tliat not a single 



