262 BEPORT— 1901. 



for public services in science are made equal to those in other branches 

 of the public service, so long will science continue to be held in insuf- 

 ficient esteem in our country, and the best and most promising of our 

 rising young men will be deterred from adopting chemistry as a profes- 

 sion. It is not so much the education of our chemists which is at fault 

 as the scientific education of the public as a whole. 



The ApiMcation of the Equilibrium Laiv to the SeiJaration of Crystals 

 from Complex Solutions and to the Formation of Oceanic Salt 

 Deposits. By Dr. E. Frankland Armstrong. 



[Ordered by the Council to be printed in cxtenso.l 



The celebrated deposits of Stassfurt consist, it is well known, of an 

 immense thickness of Rock salt, interspersed at fairly regular intervals 

 with narrow bands of anhydrous Calcium sulphate capped with beds rich 

 in Magnesium and Potassium salts. That such salt deposits are of marine 

 origin is obvious ; but as their amount is much greater than could have 

 been derived from the evaporation of the body of water present on the 

 area over which they are distributed, even supposing its depth to have 

 been that of the very deepest oceans yet known, a constant flowing in of 

 water containing salts during the period of evaporation must be assumed 

 to have taken place. As will be obvious later on, the presence of alternate 

 bands of Anhydrite and Rock salt throughout the deposits afibrds further 

 proof that such an inflow must regularly have taken place. 



Roughly, the deposits may be divided into the following four regions : 



1. Anhydrite (CaS04). 



2. Polyhalite (2CaSO^.MgS04.K2SOj.2HoO), about 60 metres thick. 



3. Kieserite (MgS04.HoO), about 30 metres. 



4. Carnallite (MgCl2.KCl.6H2O), about 23 metres. 



The presence in these deposits of salts such as Anhydrite and Kieserite, 

 which are not those normally deposited from simple aqueous solutions, is 

 in itself proof that the character of the separation is affected by the con- 

 ditions — i.e. the presence of other salts. The problem has been to deter- 

 mine the exact conditions which would give rise to such deposits. But 

 the consideration of the separation of the salts from sea-water is merely a 

 special and somewhat complex case of the more general problem involved 

 in the study of the separation of crystalline deposits from solution, 

 Avhether in the ordinary solvents familiar to the chemist or in solvents 

 such as are fused metals and silicates. 



The work hitherto done in this field has been conducted entirely by 

 van't Hoff" and his pupils, and has already been carried so far that it is 

 possible almost completely to interpret the geological phenomena afforded 

 by the Stassfurt deposits. 



The results fall under what is commonly termed the Phase rule of 

 Willard Gibbs. No difficulty can arise in understanding them when 

 graphic methods are used. 



It is before all things essential to bear in mind, in the first place, that 

 a solution can only be spoken of as saturated with a given substance when 

 the substance is present in the solid state in contact with the solution. 

 Thus, for equilibrium to exist in the case of Sodium sulphate it is necessary 

 to have the salt in solution together with the undissolved substance. The 

 phase rule is but an expression of the fact that, in the case of solutions in 



