44 ELISHA MITCHELL SCIENTIFIC JOURNAL 



We will start with an apparently complicated case of solu- 

 tions containing- three or more components. Here the phase 

 rule has brought order out of chaos. It is merely a question 

 of time to determine the conditions of existence for all the 

 possible compounds or solid solutions. The experiments of 

 van't Hoif on the Stassfurt deposits have shown what the 

 composition of the original sea was, why the salts have come 

 down as the}' have, and even the temperature at which the 

 water was evaporated. His work has furnisned the scientific 

 explanation for the methods of separation worked out empiri- 

 cally at Stassfurt and led to new methods. 



There are today many chemists who throw up their hands 

 in despair when the}' encounter a double salt which cannot be 

 recrystallized without change and yet this is a ver}- simple 

 problem with the phase rule to guide one. The phase rule 

 enables us to tell whether a given solid is a mixture, com- 

 pound or solid solution. When we reflect on the number of 

 imaginary double salts and basic salts which encumber the 

 literature, we see how sorely such a criterion has been needed 

 in the past. We are now able to attack the problems of frac- 

 tional crystallization and precipitation in a rational manner. 



Today new methods of separating rare earths are coming 

 into use; but, in the past, it has been largely a matter of 

 fractional crystallization and of fractional precipitation. 

 These methods have been slow and not very certain; but how 

 much of that is due to the man and how much to the method? 

 How many of those who have worked with rare earths could 

 take such a relatively simple problem as to separate NaCl and 

 KC1 by fractional crystallization, g-etting the whole of each 

 pure? If one cannot do that, why should one expect to make 

 rapid headwa} T in the separation of an unknown number of 

 unknown substances, which may or may not crystallize to- 

 gether as compounds or solid solutions? Here is a simple 

 instance of the confusion which may easily arise. If one 

 takes a certain solution of copper chloride, potassium chloride 

 and water, cool it to a certain temperature and filter, there 



