232 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



geological interest through being concerned in the processes of 

 mountain-building, is a problem for the geophysicist. These ex- 

 amples will serve to indicate that there may be many points in which 

 the Section of Geophysical-chemist^ may be of direct assistance, 

 through researches of its own type, in the work of other Sections. 



B. PROPERTIES OP MATERIALS IN THE INTERIOR OF THE EARTH. 



The late G. K. Gilbert has made apt reference 2 to the earth's in- 

 terior in these words: "Once it contained the forges of black- 

 smith gods; or it was the birthplace of our race, or the home or 

 prison of disembodied spirits * * *. Science now claims ex- 

 clusive title but holds it chiefly for speculative purposes." Upon 

 the Section of Geophysical-chemistry more than upon any other 

 will fall the duty of maintaining the validity of the title while 

 bringing the property into productive use. 



The outstanding difference of condition to which substances in the 

 earth's interior are subjected, as contrasted with substances a the 

 surface, is the tremendous difference in hydrostatic pressure. Re- 

 search on the properties of substances under high pressure is thus 

 of first importance in relation to the interior of the earth. While 

 we cannot at present go experimentally much beyond 12,000 mega- 

 bars, a pressure corresponding to about 45 kilometers (about 29 

 miles of depth), yet a really adequate knowledge of properties in 

 this range would give us an insight into the conditions in the in- 

 terior beside which our present knowledge is equivalent to almost 

 total ignorance. 



It is most important to know in this connection the compressi- 

 bilitjr of the substances concerned, at various temperatures, and in 

 both the liquid and the ciwstalline state, with its dependent constants 

 such as change of melting point with pressure, and effect of pressure 

 upon solubilit}^. Other important data are: The existence of new 

 polymorphic forms of substances ; the effect of pressure upon rigidity 

 and its related elastic moduli; the effect of pressure upon dia- 

 thermancy, thermal conductivity, specific heat capacity, and mag- 

 netic susceptibility; and the effect of pressure in modifying equili- 

 brium in homogeneous as well as heterogeneous systems. 



The properties mentioned in the preceding paragraph are all 

 properties of substances in equilibrium. The effect of pressure upon 

 rates of reaction and rates of diffusion and crystallization are also 

 of importance. If the planetesimal hypothesis of the origin of the 

 earth, or some modification of it, is true, it is possible that the 

 interior is very far from a state of chemical equilibrium, and that 

 redistribution of matter and energy may be going forward ac- 



2 U. S. Geol. Survey Prof. Paper S5C, 1913 (35). 



