THE MINERALOGRAPHY OF THE FELDSPARS 199 
feldspars do not form a complete isomorphous series. Secondly, it 
is not customary to regard silicon as a base-forming element. 
(CRYSTALLINE) SOLID SOLUTIONS 
The whole subject of solid solutions was opened up in a paper 
by van’t Hoff" which appeared in 1890, bearing the title of ‘Solid 
Solutions.” He said: ‘“‘If we regard a solid solution as a solid, 
homogeneous mixture of several substances, the composition of 
which can be changed without destroying the homogeneity, 
analogous to solutions in liquids as the solvent, it should not be 
difficult to cite cases which belong unconditionally in this cate- 
gory.” This definition of solid solutions gives us a concept that 
isa most valuable aid to the study of substances in the solid state. 
Again van’t Hoff defines a solution as ‘‘a homogeneous mixture, 
the composition of which can undergo continuous variation within 
the limits of its stable existence,’’”? or in other words a solid solution 
is a solid homogeneous complex of several substances, whose pro- 
portions may vary without loss of homogeneity. A solution may 
be defined in terms of the phase rule as ‘‘a homogeneous mixture 
which undergoes a change in composition in producing a new 
phase.”’? Washburn says that “‘a solution may be defined as a 
one-phase system composed of two or more molecular species.’’4 
Ordinarily the term “solution”’ has been limited to solids dis- 
solved in liquids. But recent investigation has shown that the 
conception of solutions should be extended to include gases and 
solids as actual solvents. The metallographer and the geologist 
are beginning to speak of “‘solid solutions,’ a term which has 
cleared the atmosphere and opened the way toward a better 
understanding of matter in the solid state. 
The terms “‘solid solution” and “crystalline solid solution” 
have both been used in referring to solids dissolved in solids. But 
the latter term is not as common, therefore the term ‘‘solid solu- 
tion’’ will be used in this discussion. 
tJ. H. van’t Hoff, “Solid Solutions,” Zeitschr. phys. Chem., V (1890), p. 322. 
2 As cited by J. V. Elsden, Principles of Chemical Geology, 1910, p. 116. 
3 J. L. R. Morgan, The Elements of Physical Chemistry, 1918, p. 147. 
4 Edward W. Washburn, Principles of Physical Chemistry, McGraw-Hill, 1915. 
