APPLICATION OF EQUILIBRIUM LAW TO SEPARATION OF CRYSTALS. 277 



of complexes (MgSOJ,, than is the case at g where the solution is 

 saturated with both Magnesium sulphate and Schonite, the discrepancy 

 disappears ; and, if the necessary correction could be made and the 

 vertical ordinate at c lowered accordingly, the model would afford a more 

 uniform indication of the direction of change. 



Obviously the conditions in solution are complex, especially when several 

 salts are present ; and the only phase in which the alteration is of the 

 same character throughout is that which has hitherto been left unnoticed — • 

 viz., the vapour phase. As concentration proceeds, and the dissolved salt 

 more and more asserts a mastery over the water molecules, the vapour 

 pressure necessarily diminishes — saturation with each salt corresponding 

 to a particular vapour pressure. From this point of view as the vapour 

 pressure at B (22"2 mm.) and that at c (20-9 mm.) exceeds that at 

 ci (20'4: mm.), there is clear evidence that the proportion of dissolved 

 molecules at g exceeds that at c, and that the separation takes place 

 towards G from both b and c. A model may be constructed which 

 affords a clear representation of the order in which the separations occur 

 if the differences between the vapour pressures of the A-arious saturated 

 solutions in presence of their equilibrators and the vapour pressure of 

 water (2.3'52 mm.^ be taken as vertical ordinates. The model thus con- 

 structed brings into prominence the fact that the separation of salts from 

 solution always occurs along slopes tending in one direction, and may be 

 regarded as a corrected form of the model previously considered. 



The character of this correction is shown in ^g. 4 by a thick line 

 drawn round the model at the required height. The highest point in the 

 corrected model is of course the end-point R, and the new vertical scale 

 has therefore been fixed by taking the ordinate of R to represent the 

 maximum vapour pressure difference. The 

 necessary data : — 



following table 



gives the 



A 



B 



C 



D 



E 



F 



G 



H 



J 



K 



L 



M 



N 



P 



Q 

 K 



Solution saturated with 



KCl . 



MgS0,.7H,0 



MgCL.6H„0 



KCl, kjSO^ 



K,SO^. K.,Mg(SO^).,.6H..O . 



K,.Mg(S0^)„.6H„0, MgSO^.TILO 



MgSO^.TH/O, MgSO,.fiH.,0 . 



MgSO^.eHjO, MaCl„6H,b . 



MgClj.SHoO, MgKC]3.6H..O 



MgKCl3.6H,0, KCl . ' . 



KCl, K„SO„"KoMg(S04;,.6H20 



KCl, K',Mg(Sd4)...GH.,0, MgS0j.7H.,0 



KCl, MgSO,.7H„6, MgS0^.6H.,0 



KCl, MgSO^.GHlO, MgKCl3.6H.,0 



MgS0,.6H„0, MgKClj.GHjO, MgCL, 6 



U..0 



Vapour 

 Pressure 



23-52 -Vapour 

 Pressure 



19-2 

 22-2 

 20-9 



7-7 

 19 

 21-6 

 20-4 

 12 



7-5 



7-6 

 12-7 

 18 

 13-7 

 12 

 11-9 



7-3 



4-3 



1-3 



2-6 



15-8 



4-5 



1-9 



31 



11-5 



160 



15-9 



10-8 



5-5 



9-8 



11-5 



11-6 



16-2 



Case IV. — - A reciprocal salt pair + Sodium chloride. It is 

 desirable to take this case into account as bearing on the problem 

 of the crystallisation of salts from sea water. In sea water Sodium 

 chloride is present in large excess in comparison with the other 

 sajts, and therefore is always in solution with the other salts at every 



