(151 ) 



and at all events was not answered by him. So the introduction 

 of the quantity b into the equation of state is perfectly natural — and 

 for everybody who assumes the existence of matter as real, indis- 

 pensable. But this is not, at least not in the same degree, the case 

 with the quantity a. Why should molecular attraction be a necessary 

 attribute of matter? From the idea: "matter is something that neces- 

 sarily occupies space" does not follow that matter will also have to 

 possess attraction. Perhaps we shall sooner or later learn to form 

 a conception on the nature of a molecule which involves that they 

 necessarily attract each other and learn to compute the value of this 

 attraction. Of late attempts have therefore been made to get abetter 

 insight into the nature of molecules, and they are supposed to be 

 either vibrating or permanent electrical double points 1 ). But even 

 if this supposition appeared to account satisfactorily for the molecular 

 attraction, yet it does not necessarily follow that attraction exists. 

 Then the question has changed in so far that it runs: are there 

 electrical double points in the molecule or not? It is true that in 

 my Thesis for the Doctorate (1873) p. 92, when the question occurred 

 whether hydrogen possesses a critical temperature I answered in the 

 sense of a high degree of probability, but only on a ground which 

 leaves some room for doubt, viz. that "It may be presumed that matter 

 will always have attraction". It is not to be denied that everything that 

 we accept as matter is subjected to gravity; but to derive from this 

 that the existence of the Newtonian attraction involves the possession 

 of molecular attraction is more than hazardous. All this is not 

 intended to raise doubt about the existence of a e. g. for helium, 

 for now that all other substances possess a value for a we may 

 repeat what I said before for hydrogen, but to draw attention to 

 the fact that the value of a does not only depend on the molecular 



weight. If this was the case, — - = — -, a relation which is cer- 



tainly not fulfilled. Then also — — = — - = — — , and ^,,' = ^0, 



which very probably will never be fulfilled either. 



I was convinced from the outset that we should not be able to 

 explain a number of phenomena occurring for binary mixtures by 

 means of such an unfounded supposition. Already a long time ago 

 Korteweg showed in his paper "La surface ip dans Ie cas de symétrie" 

 in how high a degree the phenomena exhibited by a binary mixture, 

 depend on the value ascribed to a lt . For values of a between 



] ) See these Proc. p. 13:2. 



