280 HAROLD L. ALLING 
be made to mix in all proportions while in the molten condition, it 
is highly probable that, in the great majority of cases, they can 
be mixed with one another in the fluid state in any relative propor- 
tion. In this respect these fluids resemble such liquids as water 
and alcohol. We may, in fact, safely carry this analogy much 
further, and regard mixtures of two molten silicic salts as simple 
solutions of the two constituents in one another. Since our 
interest naturally centers in the solid mineral which results from 
the solidification of such melts the question which lies before us is 
what happens to a mutual solution of two silicic salts when the 
temperature is lowered so that the material undergoes solidification ? 
The answer is that there are two opposite modes of solidification 
adopted by such systems, and a range of intermediate modes con- 
necting these extremes. The one extreme is (a) the case in which 
on solidification the mineral crystallizes while still remaining a 
solution, i.e., the crystals which are formed ultimately attain the 
same composition as the molten liquid from which they crystallize. 
Such crystallized solutions are usually termed “‘solid solutions” in 
view of the fact that they are at the same time solids and solutions. 
This is the case when perfect isomorphism exists. 
Such solid solutions should not be regarded as compounds, and 
no single chemical formula can be employed to express the mineral 
as a whole. 
The other extreme of the mode of solidification (6) is that in 
which the state of solution which exists in the liquid condition is 
largely or entirely destroyed by the passage into the solid state, 
the two constituents separating more or less completely during the 
process of crystallization. This condition occurs when limited 
isomorphism prevails. 
THE EQUILIBRIUM DIAGRAM 
The most comprehensive and satisfactory method of presenting 
and describing the nature and constitution of minerals belonging 
to a given system consists in a diagram—the thermo-equilibrium 
diagram—which is based primarily upon thermal data. The con- 
struction of such a diagram is based upon the determination of the 
temperature of the specimen at various times during a heating or 
cooling process. The usual method consists in taking temperature 
