673 



Above the temperature of tlie higher cnsp e, and below the pressure 

 of the lower cusp d, the solid substance can no- longer exist by 

 the side of the gas resp. liquid. 



For our purpose the cusp c is tlie most important, for this point 

 expresses that there exists a limit for the coexistence solid -[- gas, 

 which implies that the orientated condition of the molecules in the 

 solid substance coexisting with its vapour cannot exist an}' longer 

 at a definite temperature, in consequence of the increasing molecular 

 motion. If this holds for the solid substance in coexistence with 

 vapour, there must also be a limit of existence for the solid substance 

 ivithoiit vapour, and as the contact with the vapour will diminish 

 the stabilit}^ of the solid state in consequence of the molecular 

 attraction between the molecules in the solid phase and those in the 

 gas phase, we may expect that the limit of existence of the solid 

 substance ivitliout vapour will lie at a higher temperature. This 

 temperature limit of existence will vary with the pressure, and thus 

 we shall be able to draw a line uv in the PT-figure indicating the 

 limit of eMstence of the solid substance. 



For a binary system this holds of course, for both the components. 

 When, therefore, we pass from the triple point of the components 

 to the quadruple point, we get something similar. As Dr. Scheffer 

 has demonstrated ') and as is expressed in Fig. 2, the lines for 

 Sa -\- Sb -\- 0. Sa -\- Sb-\- L merge continuously into each other 



Fig. 2. 



also by means of a ridge with two cusps and an unstable inter- 



1) These Proc. XIII p. 158. 



