422 Itqjorts and Proceedings — British Association. 



As is well known, the modern theories of solution mainly rest upon 

 the behaviour of dilute solutions from which the principles of 

 electrolytic dissociation have been deduced ; but in the case of the 

 concentrated solutions from which dissolved substances actually 

 crystallise, very little is really known about the liquid itself. A great 

 deal is known, however, about its equilibrium with the solids that 

 separate from it, and the general laws of this equilibrium are 

 expressed by the phase-rule deduced from mathematical considerations 

 by Willard Gibbs, which states how many mechanically separable 

 constituents can coexist under varying conditions of equilibrium in 

 a system containing a definite number of chemical components. 



A solution saturated with a given substance is one which is in 

 equilibrium with that substance when the latter is in contact with it 

 in the solid form ; the phase-rule indicates the number of solids 

 which must be in contact with a given solution ; the only difficulty 

 in practice is to determine the nature of the double salts or distinct 

 hydrates that may be formed. 



By means of a series of experiments upon the solubilities of these 

 salts, either singly or in the presence of one another, in order to 

 determine the composition of solutions saturated simultaneously with 

 two or more substances, it is possible to obtain a graphic repre- 

 sentation of all possible solutions containing the salts present in 

 sea-water. From this the course of crystallisation of any particular 

 solution, for example sea-water, can be predicted. 



The general sequence thus theoretically predicted is as follows : — 

 (1) Kock salt ; (2) Eock salt with the magnesium sulphate, epsomite ; 

 (3) Kock salt with the double sulphate of potassium and magnesium, 

 leonite ; (4) Eock salt with leonite and the potassium chloride, 

 kainite ; (5) Eock salt, the magnesium sulphate kieserite, and the 

 double chloride of potassium and magnesium, carnallite ; (6) Eock 

 salt, kieserite, carnallite, and the magnesium chloride, bischoffite. 

 This last combination will persist until all the water is evaporated. 

 This is found to be the general sequence, not only of the salts 

 obtained on evaporating sea-water at 25°, but also of the Stassfurt 

 deposits. 



Up to this point the results have been summarised by Dr. E. F. 

 Armstrong in a report presented to the British Association in 1901. 

 Since that date the research has been prosecuted actively by van 't 

 Hoff and his pupils, and now the conditions of equilibrium at 25^ 

 have been mapped out, not only for the above compounds, but also 

 for the minerals thenardite, glaserite, astrakauite, and reichardtite, 

 which occur in these deposits. The whole process of crystallisation 

 of the solution, from which no fewer than twelve difi'erent salts have 

 separated, can, therefore, be predicted, and their sequence and 

 associations can be traced through numerous stages, beginning with 

 the separation of rock salt and ending with a mixture of rock salt, 

 kieserite, carnallite, and bischoffite. 



In reconstructing the history of these deposits account must also 

 be taken of the varying vapour pressures of the solutions which are 

 saturated with the different compounds, as this really determines 



