39 



this pseudo-system, a,s well with regard lo Ihe gaseous as to the 

 liquid and the solid phase. 



Tlie phenoineuou, observed here with great probabilily, of iiiter- 

 luil equilibriuui between sirticture isomers will be of j)i-etty general 

 occurrence without any doubt, so that it will not be difficnlt to 

 discover many more interesting examples, which are accessible to 

 experiment. 



Anorg. Chem. hab. of the University . 



Amsterdam, May 27, 1913. 



Physics. — "Oil tlw point in inhidi. the solid date disuppears as 

 an answer to the (fiiestion in hoin far this point can be compared 

 to the critical point of a liquid. The easiest way to do this 

 is by means of the yip-curve" . By Prof. J. D. van der Waaj.s. 



(Communicated in the meeting of Febt'uai'y 22, 1913). 



If the ifj-curve is drawn at the triple point, the same straight line 

 is touched by : 1. the ipcurve li(|uid-vapour, 2. the if>curve for the 

 solid substance. Let us put the case that occurs most frequently, in 

 which Vs <^ vi <[ V(j. On rise of the temperature the if>curves descend, 

 but not in an equal degree. The liquid-vapour curve descends more 

 than the curve for the solid substance. Relatively ip., ascends there- 

 fore. And accordingly the tangent for the coexisting states liquid- 

 vapour, and that for the coexisting states lic|uid-solid are separated. 

 The pressure has increased, but that foi" the coexistence solid-liquid 

 far more than for the coexistence liquid-vapour. When the tenq:)e- 

 rature continues to rise the ip-curve for the liquid will more and more 

 approach the i^-curve for the solid substance, and it will reach it 

 at the temperature at which the solid state disappears at infinite 

 pressure. At the critical point liquid- vapour these two states are 

 identical; the solid state on the other hand has been expelled by 

 the liquid state. This, however, takes place at a pressure equal to 

 intinity and so a volume equal to v^. Above this temperature the 

 solid state no longer exists, but the liquid state does. I have shown 

 elsewhere, however, that then, when the pressure again aj^proaches 

 infinity, and hence the volume to Vo, the viscous-solid state will 

 probably appear. 



If we now consider the case, in which, as for water and ice 

 Vs'^vi, the 29,7'-line for the coexisting phases runs from the triple 

 point to lower temperatures. Now two cases are possible, viz. that 



