( 3Ü0 ) 



be tlie case, a plait, wliich appears as a longitudinal plait, but 

 ■which has its plaitpoint on the side of the small volumes, as in 

 tlie observations of Kuenen and Robson, is not a longitudinal plait, 

 but a somewhat modified transverse plait. If then in the liquid 

 state complete mixture does not take place under all circumstances, 

 the phenomenon is to be ascribed to other causes than in case of 

 a real longitudinal plait. 



If further investigations confirm what has been said, we should 

 have to find the cause in this case in the fact that at least one 

 of the substances is abnormal. 



Physics. — "0/« variation of volume and of pressure". III. By 

 Prof. J. D. VAN DER Waals. 



In order to be able to judge about the extent of the variation of 

 volume and of pressure in mixtures of two substances, it will be 

 necessary, to find for the different proportions of mixing the pres- 

 sures at which the number of the molecules is the same for the 

 two substances, if the volume and the temperature are also the 

 same. If there were no deviations from the law of Boyle, the pres- 

 sures required would be equal. Assume that for a mixture the cha- 

 racteristic equation is : 



at a given temperature the 2"'^' member of this equation has a con- 

 stant value for mixtures with the same number of molecules, and 

 so we may find the pressure which satisfies the preceding condition 

 in solving the value of p from this equation, if the value of v is 

 constant. Take as unity of pressure 1 atmosphere and as unity 

 of volume, the volume occupied by a molecular quantity of the 

 mixture under that pressure. The value of the second member is then 



(1+a,) (1-M (1 4 «0, 

 for which we may write with a sufficient degree of approximation 

 (1 +a.,-?^x) (1 +«0- 

 If we write p^ for the value of the pressure taken as unity, and 



