200 HAROLD L. ALLING 
Solid solutions are frequently referred to as ‘‘mixed crystals,” 
a term probably derived from a too literal translation of the 
German ‘‘Mischkrystalle.”’ This translation is unfortunate since 
the term may suggest a merely mechanical mixture of crystalline 
substances like sand. As the term is liable to convey an entirely 
erroneous impression, it should be avoided in the interest of 
clearness. 
Owing to the restricted nature of molecular motion in crystals and the high 
viscosity of this state of aggregation, crystalline solutions in a state of equi- 
librium are seldom met with in practice, because the attainment of equilibrium 
in a reasonable length of time is so frequently prevented by the restraints upon 
the free movements of the molecules. 
This fact prevents satisfactory laboratory work on the problem. 
In geology we have at our disposal enormous lengths of time in 
which perfect equilibrium may be established, a factor, the impor- 
tance of which is not always appreciated and which of course is 
rarely attained in the laboratory. 
Although the writer is inclined to the view that the feldspars 
are aluminosilicates which in many cases are capable of forming 
solid solutions, nevertheless the usual nomenclature (ortho-, 
meta-, and trisilicates) will be employed without necessarily 
implying any particular theory of the chemical nature of these 
minerals. 
ISOMORPHISM AND SOLID SOLUTION 
Recognizing that isomorphous substances must be completely 
soluble in each other some mineralogists have come to the con- 
clusion that the terms “isomorphism” and ‘‘solid solution” are 
synonymous. As this matter is of some importance here it will be 
discussed. 
“The establishment of complete solid solution between the 
[plagioclase] feldspars raises the whole question of the use of 
the terms solid solution and isomorphism. Some authors use 
isomorphism to designate complete solid solution, others speak 
freely of limited isomorphism, and still others use the term in its 
original significance of simple crystallographic similarity.’” 
1. W. Washburn, Principles of Physical Chemistry, 1915, Pp- 117. 
2.N. L. Bowen, Amer. Jour. Sci. (4), XXXV (1913), 595. 
