HIGH-PRESSURE PHYSICS — BRIDGMAN 205 



the application of powerful forces other than hydrostatic pressure. 

 Most apparent exceptions can be explained by the closing of flaws 

 in the materials. There are some other apparent exceptions, due to 

 a phase of greater thermodynamic stability than the original being 

 produced by pressure. In such cases, however, the new phase created 

 shows no permanent change when pressure is again applied and re- 

 leased. The reason for the failure of hydrostatic pressure to produce 

 permanent changes is to be sought in the atomic constitution of 

 matter. The specific volume of a substance depends only on the 

 nature of the atoms (or molecules) of which it is composed ; neglect- 

 ing isotopes, there is only one kind of iron atom, for example, not 

 one kind before it has been compressed and another kind after- 

 ward. This suggests that if pressures could be applied which were 

 high enough to change the atoms themselves we might expect per- 

 manent changes of volume. Ordinarily, however, atomic transmuta- 

 tion is not brought about by pressure only, and we therefore have 

 perfect volume elasticity. 



In the realm of terrestrial pressures, within which the atoms remain 

 unaltered, at least three classes of effect must be considered. In the 

 first place there is the gaseous range. Here compressibilities are very 

 high, volume being approximately inversely proportional to pressure. 

 The mechanism is a kinetic one, pressure being exerted by the collision 

 of the molecules with the walls of the container. Pressure is twice as 

 high at half the volume because there are approximately twice as many 

 collisions per unit area of the walls. This effect cannot, however, per- 

 sist over any considerable range of pressure, because eventually the 

 molecules begin to interfere with each other through being pushed too 

 closely together. When this happens a new effect occurs. Increasing 

 pressure now pushes the molecules progressively more closely together 

 until all the empty spaces between them are squeezed out and the mole- 

 cules are effectively in contact. This is the sort of thing that occurs in 

 the compression of liquids under ordinary conditions, or in the com- 

 pression of gases under such high pressures that their density ap- 

 proaches that of liquids. This effect is characterized by a compressi- 

 bility that falls off rapidly with decreasing volume, for at first the 

 empty spaces can be squeezed smaller with comparative ease, but when 

 the molecules are nearly in complete contact this possibility is greatly 

 reduced. The third effect now begins, namely, the deformation of the 

 molecules or atoms themselves after they have been squeezed into effec- 

 tive contact. It is with this that we are primarily concerned in the 

 compression of solids. Modern knowledge of the atom as a system of 

 electrons and nucleus, depicting the atom as consisting almost entirely 

 of empty space pervaded by an intense field of force, makes compre- 

 hensible a high degi-ee of deformability under pressure in the atom it- 



