8o PHYSICAL SCIENCE 



the liquid, can vary continuously in composition ; 

 the other, or solid, phase is fixed and invariable. 

 Similarly in the case illustrated in Fig. 8, the 

 crystals of the compound SbCug have a fixed 

 and constant composition. Cases are known, 

 however, in which the solid phase also varies 

 continuously. Many salts, such as the different 

 alums, are of the same crystalline form, and can 

 replace each other gradually in a crystal, which 

 may have any composition between that of the 

 two pure salts. Such structures are called mixed 

 crystals or solid solutions. When they can exist, 

 the phenomena of equilibrium become much more 

 complicated, for the composition of the solid will 

 vary as well as that of the liquid, and will in- 

 troduce a second curve into the freezing-point 

 diagram. 



It is only of recent years that it has been 

 possible to interpret the complicated phenomena 

 of solid solutions. Now, however, we possess a 

 consistent theory of the subject, founded by 

 Professor Roozeboom of Amsterdam, on the work 

 of the late Professor Willard Gibbs of Yale 

 University. Long ago, in the years 1875 ^^ 

 1878, Gibbs published a series of mathematical 

 papers in the Transactions of the Connecticut 

 Academy. For some time they remained practi- 

 cally unknown to European physicists ; then they 

 were discovered by Clerk Maxwell, who used a 

 few of the results in his book on the *' Theory of 

 Heat." But even then the time was not ripe, and 

 it is only of recent years that we have realised 

 that the whole theory of chemical and physical 

 equilibrium is contained in Gibbs' work. Buried 

 for so long, the seed has germinated in the minds 



