APPLtCATiON OF EQUILIBRIUM LAW TO SEPARATION OF CRYSTALS. 269 



Carnallite are present in the region ODC. The difference between the 

 regions OBC and odc consists in the fact that ih the former Carnallite, 

 and in the latter Magnesium chloridej predominates, one or other of these 

 salts, as the case may be, separating when the solution is concentrated. In 

 both cases the composition of the solution tends towards that represented 

 by the point c. When this point is reached Magnesium chloride and 

 Carnallite are deposited together in constant proportions, the solution 

 evaporating to dryness without further change of composition. It will 

 hence be obvious that the point c is one of critical importance, as defining 

 the conditions under which the final crystallisation takes place. It is 

 termed by German workers the ' Krystallisations Endpmikt ' — the ter- 

 minus of crystallisation. The determination of such points is the object 

 in view in discussing a problem such as that afforded by the Stassfurt 

 deposits. 



It is, however, necessary to make one more stipulation in order to 

 render the previous statements universally true — viz., that the regular 

 sequence of crystallisation may not be followed unless the product which 

 separates is periodically removed from contact with the solution. If this 

 be not done, secondary action may take place, and the product at first 

 formed may be eaten up again by the solution. For example, if after 

 reaching the point b the deposited Potassium chloride be not removed, 

 on further concentration, as two equilibrators are present, the solution will 

 evaporate without changing its composition ; but as a large excess of 

 Magnesium chloride is present, and this gradually comes into operation 

 as water is removed. Potassium chloride will be continually re-dissolved 

 (42-5 mols. K2CI2 per 100 mols. Carnallite deposited). As soon as all 

 solid Potassium chloride is removed, the deposition of Carnallite causes 

 the composition of the solution to change until the ' end-point ' c is 

 reached. In interpreting such diagrams, therefore, it is to be assumed 

 that the products deposited are removed from solution at the proper 

 moment. It may be supposed that this often takes place in nature through 

 the deposition of a protecting layer of mud. 



Case III. — Eeciprocal salt jxiirs. As an example may be taken the 

 reciprocal salt pair which is of greatest importance in the investigation of 

 sea water — i.e., that represented by the equation 



MgCl^ + K^SO^ ;t K.Cl^ + MgSO^. 



These salts give rise to two double salts, and at least two hydrates of 

 MgSO., have to be considered ; therefore it is necessary to determine the 

 composition of the saturated solutions of the stable combinations of seven 

 substances, taken 



(a) Singly, 



(6) In pairs, 



(c) Three at a time. 



The table on page 270 shows the composition of the various solu- 

 tions fulfilling the conditions of equilibrium. 



Considering the table in detail, in the case of solutions saturated 

 with a single salt it is only necessary to point out that the Potassium 

 chloride is expressed in double molecules, as a system of equivalent nota- 

 tion must be used. The meaning of the figures appended to the solutions 

 saturated with two salts is in most cases at once apparent, but tlie solution 

 H requires a few words of explanation, as the equilibrators in this case 



