August i, 1901] 



NA TURE 



337 



sodium chloride in the solution increases, and the composition 

 of the solution is represented successively by points on dc At 

 c sodium chloride begins to crystallise and the two chlorides 

 are then deposited in the proportions determined by the position 

 of c on the diagram until the evaporation is complete. The 

 arrows indicate the course of crystallisation for any given 

 solution. 



The phenomena of crystallisation may be somewhat more 

 complicated if the two salts in solution are capable of double 

 salt formation. Whether the double salt crystallises out on 

 evaporation depends essentially upon the temperature. For 

 e.Kample, the evaporation of a solution containing the chlorides 

 of calcium and magnesium yields a mixture of the .simple salts 

 below 22° C, whereas at a higher temperature the double salt, 

 tachhydrite, CaClo.2MgCl2. I2H.,0, accompanied in general by 

 one of the simple salts, crystallises out. 



Let us consider the crystallisation of a solution containing 

 the sulphates of magnesium and potassium at a temperature of 

 25° C, this lying between the limiting (transition) temperatures 

 at which the double salt, schonite, K2SO4.MgSOj.6H.2O, is 

 formed and decomposed. 



The solubility data which furnish us with fixed points by 

 means of which the course of crystallisation is determined are in 

 this case four, viz., the saturated solutions of (l) JIgS04.7H„0, 

 (2) KoSOj, (3) schonite and MgS04.7H„0, (4) schonite and 

 K2SO4. 



Mi.C( 



In Fig. 2, where potassium sulphate is measured off on the 

 abscissa and magnesium sulphate on the ordinate, these solutions 

 are represented respectively by the points A, B, c and D. 

 The connecting curves have the same significance as in Fig. I. 



Suppose we slowly evaporate at 25° C. a solution the com- 

 position of which is given by the point a, which lies on the line 

 bisecting the angle between the axes ; this solution obviously con- 

 tains equivalent quantities of potassium and magnesium sulphates. 

 The inde.\ point representing successive conditions of the solution 

 advances along the continuation of o a until the point i is reached, 

 when the solution becomes saturated with potassium sulphate. 

 As evaporation continues and crystallisation of potassium sulphate 

 takes place we advance along b D until at the latter point the 

 solution becomes saturated with regard to schonite. On further 

 loss of water schonite crj'stallises out, and since the molecular 

 concentration of magnesium sulphate in the solution is greater 

 than that of potassium sulphate, the continued separ.ition of 

 the double salt increases the molecular ratio MgSOj : K2SO4 in 

 the solution, corresponding to a movement of the index point 

 along D c, the curve of saturation of the double salt. If the 

 solution were agitated so as to bring the separated K2SO4 into 

 intimate contact, some of the latter would again pass into the 

 solution and reappear as schonite. At the point c the solution 

 becomes saturated with regard to MgS04. 7H2O, and Epsom 

 salts and schonite now crystallise out together until the solution 

 completely disappears. The point c represents the crystallisa- 

 tion end point of all solutions containing magnesium and 

 potassium sulphates, the final separation from all such solutions 



NO. 1657, VOL. 64] 



being a mixture or schonite and Epsom salts. The arrows in 

 the figure show the course of the crystallisation for any solution. 

 If we now suppose that one of the two salts is capable of 

 dehydrating the other when the solution becomes concentrated, 

 the products of crystallisation which are first separated may 

 undergo a series of successive transformations. If the two salts 

 in solution do not unite to form a double salt, as in the 

 case of magnesium sulphate and magnesium chloride, the 



phenomenon to be discussed appears in its simplest form. It 

 may be pointed out that the temperature at which any hydrate 

 is transformed into a lower hydrate is lowered if foreign sub- 

 stances are added to the aqueous solution with which the two 

 hydrates are in equilibrium at the transition temperature, just in 

 the same manner as the melting point of a pure substance is 

 lowered by foreign admixtures. Consequently we may expect, 

 at one and the same temperature, higher or lower hydrates to 

 crystallise out from a solution according as the mother liquor 

 contains small or large quantities of other soluble substances. 



Fig. 3. 



The application of the.se known facts to the crystallisation of 

 a solution containing the sulphate and chloride of magnesium 

 at 25° C. enables us to explain -completely the successive changes 

 observed. 



Representing, as before, the composition of the saturated 

 solutions of these salts on a system of coordinates. Fig. 3 

 is obtained. .4 and B represent the saturated solutions of 



