( 138 ) 



the viria! of the resulting mean molecular attraction need only be 

 taken into account. It is true that the molecules exercise on each 

 other still other forces besides this mean attraction, but these forces 

 yield a virial zero. The forces normal to the radius vector, namely 

 form together a couple, and the virial of a couple is zero. For the 

 same reason the directing couples working on the molecules need 

 not be taken into account. And from the forces working in the 

 direction of the radius vector we need only take into account the 

 average value, for attracting and repulsing forces which equally 

 often occur between different pairs of molecules, cancel each other. 

 In calculating the virial, the influence of the molecular attraction 

 on the distribution in space of the molecules must of course still be 

 taken into account. 



Chemistry. — "Equilibria in quaternary systems.' 



F. A. H. SCHREINEM AKERS. 



By Prof. Dr 



In the system: Copper sulphate, ammonium sulphate, lithium sul- 

 phate and water two more solid compounds occur at 30° in addition 

 to the three sulphates namely, Cu S0 4 (NH 4 ), S0 4 , 6 H a O and Li 2 S0 4 

 (NH 4 ) 2 S0 4 . 



We will again represent the equilibria in the wellknown manner 

 with the aid of a tetrahedron but now choose quite a different pro- 

 jection than that used in the previous communication; we will in 

 fact project all saturation lines and surfaces perpendicularly on one 

 of the side planes of the tetrahedron. 



A projection of this kind is represented in the figure; the points 



