Some Results in the Field of High-Pressure 



Physics^ 



By P. W. Bridgman 

 Department of Physics, Harvard University 



In this article I shall describe some of the physical phenomena that 

 are produced in various materials when exposed to high hydrostatic 

 pressures. Pressure is not usually thought of as having any impor- 

 tant effect on the properties of materials, and indeed under the ordi- 

 nary conditions in which human beings live it does not. The effect 

 of temperature is usually far more important for us, for temperature 

 changes may produce such important results as the conversion of ordi- 

 nary liquid water to solid ice or gaseous steam. One reason for the 

 comparative unimportance of pressure is that the variations of pres- 

 sure that we can easily produce are, so far as the molecules are con- 

 cerned, not large. Under pressures that are large for the molecules, 

 changes may be produced quite as drastic as those brought about by 

 changes of temperature. For example, water may not only be frozen 

 solid by the application of pressure alone, but pressure is capable of 

 producing seven different kinds of ice, something that mere alteration 

 of temperature is unable to accomplish. 



The pressures that are large enough to affect molecules are in gene- 

 ral of the order of thousands of atmospheres, and it is with such pres- 

 sures that we shall be concerned here. To set the scale, a thousand 

 atmospheres, or some 15,000 pounds per square inch, is approximately 

 the pressure at the deepest part of the ocean, produced by a column 

 of water G miles high. Two thousand atmospheres is approximately 

 the pressure in the explosion chamber of a large gun. From the cos- 

 mic point of view, the importance of understanding the effects of 

 pressures of tliis magnitude is obvious, because all except a small frac- 

 tion of 1 percent of the matter in the universe exists under pressures 

 greater than 1,000 atmospheres. 



In extending scientific measurements into the realm of pressures of 

 this magnitude, various technological problems are encountered. 

 There is in the first place the problem of preventmg leakage of the 

 liquid by which pressure is transmitted. This problem may be solved 



^ Reprinted by permission from Endeavour, vol. 10, No. S8, April 1951. 



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