( 139 } 



Til order to dmw attention to tlie pnncij)lc circumstances, we 

 shall assume for the presenl thai the following- eciuations also hold 

 in the case that h is \'ariablc : 



and 



Ö a 



RTk = 



27 6, 



^^ 27 hg"" 



Equation (5) may then be written in this form : 



A comparison of this equation with : 



shows that it is possible to satisfy the condition that the coefficient of 



— approaches to 7 by equating -- to 2, i.e. by assuminp: that the 



molecules in volumes equal to the volume of liquids at low tempe- 

 ratures are only half as large as those in the gaseous condition. IJut 



the agreement in the value of the coefficient of -7^ does not suffice for 



establishing agreement between the calculated value and that of 

 the formula which at low temperatures is followed by the \apour 

 tension, though it be only in large features. For this purpose it is 

 required that 



ditfers only slightly from 7. 



We must return to the equation of state in order to be able to 



determine the value of this expression, and Ave must investigate its 



consequences for the case that /> may be neglected compared with 



a 



— . So we must return to : 



a 8 a T 



V-' ^' '^ 27 h, n 



and 



If we express ö^ and v^ in the quantity j, we get 



1-z' 



