day: geophysical research 255 



pose it to have passed thru in the earth, provided only we « in 

 reproduce that condition in the laboratory. 



During the past quarter of a century, there has arisen in the 

 middle ground between physics and chemistry a new science of 

 physical chemistry, in the development of which generalizations 

 of great value in the study of minerals have been established. 

 As long ago as 1861 the distinguished German chemist, Bunsen, 

 pointed out that the rocks must be considered to be solutions 

 and must be studied as such; but inasmuch as comparatively 

 little was known about solutions in those days, and the rocks 

 at best appeared to be very complicated ones, no active steps 

 in that direction were taken during Bunsen's life. But in recent 

 years solutions have been widely studied, under rather limited 

 conditions of temperature and pressure, to be sure, but it has 

 resulted in establishing, relations — like the phase rule — of such 

 effective and far-reaching character, that now, just half a century 

 afterward, we are entering with great vigor upon the prosecution 

 of Bunsen's suggestion. It is now possible to establish definite 

 limits of solubility of one mineral in another, and definite condi- 

 tions of equilibrium, even in rather complicated groups of min- 

 erals, which enables us not only to interpret the relations devel- 

 oped by such a thermal study as that outlined above, but also 

 to assure ourselves that only a definitely limited number of com- 

 pounds of two minerals can exist, that they must bear a constant 

 and characteristic relation to each other under given conditions 

 of temperature and pressure, and that changes of temperature 

 and pressure will affect this relation in a definite and determinable 

 way. Physical chemistry not only takes into account the chem- 

 ical composition of mineral compounds, but their physical prop- 

 erties as well, thruout the entire temperature region in which 

 they have a stable existence, and therefore furnishes us at once 

 with the possibility of a new and adequately comprehensive classi- 

 fication of all the minerals and rocks in the earth. The value of 

 an adequate system of classification appeals chiefly to those whose 

 duties bring them into intimate relations with the subject matter 

 of a science, but so much may appropriately be said, that a con- 

 sistent application of physical chemistry to the minerals may 



