GEOPHYSICAL RESEARCH — DAY. 365 



mation, the temperature region within which the new forms are stable, 

 and the changes which each undergoes with changes of pressure and 

 temperature, as before. If the new forms show signs of instabihty, 

 we can drop them into cold water or mercury so quickly that there 

 will be no opportunity to return to initial stable forms, and thus 

 obtain, for study with the microscope at our leisure, every individual 

 phase of tke process through wliich the group of minerals has passed. 



Without complicating the illustration further, it is obvious that 

 we have it m our power to reproduce in detail the actual process of 

 rock formation within the earth, and to substitute measurement 

 where the geologist has been obhged to use inference; to tabulate 

 the whole history of the formation of a mineral or group of minerals 

 under every variety of condition which we may suppose it to have 

 passed through in the earth, provided only we can reproduce that 

 condition m the laboratory. 



During the past quarter of a centuiy there has arisen in the middle 

 gi'ound 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, pomted out that 

 the rocks must be considered to be solutions and must be studied as 

 such; but, inasmuch as comparatively little was known about solu- 

 tions in those days, and the rocks at best appeared to be very com- 

 plicated ones, no active steps in that dhection were taken durmg 

 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 m establisMng relations — like the fliase rule — 

 of such effective and far-reacMng character that now, just half a 

 century afterwards, we are entering with great \dgor upon the prose- 

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

 limits of solubility of one mineral in another, and definite conditions 

 of ec[uilibriuni, even in rather comphcated gi-oups of minerals, which 

 enables us not only to interpret the relations developed by such a 

 thermal study as that outlined above, but also to assure ourselves 

 that only a definitely limited number of compounds 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 afl'ect this relation 

 in a definite and determinable way. Physical chemistry not only 

 takes into account the chemical composition of mineral compounds, 

 but then- physical properties as well, throughout the entire tempera- 

 ture region in which they have a stable existence, and therefore fur- 

 nishes us at once with the possibility of a new and adequately com- 

 prehensive classification of all the minerals and rocks in the earth. 

 The value of an adequate system of classification appeals chiefly to 



