( 819 ) 



jeetion on a plane parallel to two sicU^s erossing each other, in 

 this case Ihe sides: 11'.] and A()N, so that in Ihe projection, these 

 stand perpendicular to each olher and divide each other in two. 



H'^'--'- 



Fig. 2. 



The angular points W, A, Ag and .V indicate the components 

 water, ethyl alcohol, silver nitrate and animoninni nitrate. The pro- 

 jection of an arbilary i)oint within (he tetrahedron on the projection 

 plane is easily indicated. If we take the line WA as A-axis and 

 the line NAy as F-axis of a co-ordinate system and if we take as 

 positive directions those towards A and Ag we find: 



X = 



A—W 



r= 



Ag-N 



when A, W, Ag and A' represent the quantities of alcohol, water, 

 silver nitrate and ammonium nitrate indicated by the said point 

 withiji the leti-ahedron. 



In this maimer Fig. 2 has been deduced and it is readily noticed 

 that (he equilibria are represenled by three surfaces, namely s.s\)\r, 

 r i\ (j^ q and q q^ p^ p. The first surface is the saturation surface of silver 

 nitrate, the second (hat of (he double salt and the (hird (ha( of (he 

 ammonium ni(rate. 



The double saU is represented in the figure by the point Z) which, 



