HIGH-PRESSURE PHYSICS — BRIDGMAN 211 



known substance is proportional to the pressure. The significance of 

 this in geophysics is obvious; it means that the probabihty is that the 

 materials in the earth's crust occur there in forms with which we are 

 not familiar in the laboratory, and therefore have unfamiliar physical 

 properties. This implies that it is hazardous to infer the composi- 

 tion of the earth's crust from such evidence about its properties as is 

 given by the velocity of propagation of seismic disturbances. Un- 

 certainty with regard to inferences of this sort can be removed only 

 by specific and detailed knowledge. 



The transitions discussed thus far have been characterized by 

 thermodynamic reversibility ; when the pressure is removed the sub- 

 stance reverts to its original condition, unless by chance it is hindered 

 at comparatively low temperatures by internal viscosity. In addition 

 to these reversible transitions there are a few examples known of 

 essentially irreversible transitions produced by pressure. Here the 

 substance is converted into a new form by the application of pressure ; 

 this new form is then permanently retained and is thermodynamically 

 stable when the pressure is removed. The transformation of yellow 

 phosphorus into black by a pressure of 12,000 atmospheres at 200° C. 

 is an example that has been known for a long time. The same transi- 

 tion may be made to occur at room temperature by 30,000 or 40,000 

 atmospheres. At still higher pressures — 60,000 atmospheres or so — 

 the more stable red or violet phosphorus may be transformed into the 

 same black variety. Another example is carbon disulfide. This sub- 

 stance, ordinarily a highly volatile liquid, is slowly transformed at 

 175° C. and 40,000 atmospheres into a permanent black solid, denser 

 than the elements from which it is constructed. 



The theory of these irreversible transitions is even less well under- 

 stood than that of the reversible polymorphic transitions, and indeed, 

 as far as I know, no example has ever been worked out theoretically. 

 Until we have some theoretical basis for knowing what to expect, 

 we must contemplate the possibility that any of the materials of 

 daily life can, by sufficient pressure, be pushed over a potential hill 

 into some entirely unknown form possessing new, and perhaps desir- 

 able, properties. 



