Vol. 6, 1920 
GEOPHYSICS: R. B. SOSMAN 
599 
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 megabars, a pressure corresponding 
to about 45 km. (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 interior beside which our present knowledge is equivalent to almost 
total ignorance. 
It is most important to know in this connection the compressibility of 
the substances concerned, at various temperatures, and in both the liquid 
and the crystalline state, with its dependent constants such as change 
of melting-point with pressure, and effect of pressure upon solubility. 
Other important data are : the existence of new polymorphic forms of sub- 
stances ; the effect of pressure upon rigidity and its related elastic moduli ; 
the effect of pressure upon diathermancy, thermal conductivity, specific 
heat capacity, and magnetic susceptibility; and the effect of pressure in 
modifying equilibrium 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 reac- 
tion and rates of diffusion and crystallization are also of importance. If 
the planetesimal hypothesis of the origin of the earth, or some modifica- 
tion 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 actively, with what results — whether rising or falling 
average temperature, increasing or decreasing volume, etc. — it is impossi- 
ble now to say. 
The preceding considerations apply also to the larger units of struc- 
ture of the interior — although what these units may be we can only guess — 
and also to the earth itself as a unit. Complex mathematical analysis 
is necessary in applying the data to the units of structure as well as to 
the whole of a body as large as the earth, where the force of gravitation 
is itself variable with depth. These applications can in many cases be 
best made by those familiar with the properties in question as measured 
in the laboratory. The course of earthquake waves, to take an example 
of interest to the Section of Seismology, is dependent both upon the 
elastic constants of the earth's materials, and the possible reflection and 
refraction of waves at the boundaries of internal structural features. 
In this connection, the members of this Section could do a service to 
the other Sections by making clearer the real meaning of some of the 
physical concepts and physical constants involved in geophysical problems. 
For instance, much confusion has been caused by the fact that there is 
more than one kind of "rigidity". The geologist to whom the state- 
ment that "the earth has the rigidity of steel" is rather vague may take 
temporary comfort from the fact that the statement also needs much quali- 
fication and explanation to the physicist.^ 
