206 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



self. According to this picture, atoms with the most complicated elec- 

 tronic structures should be capable of a higher degree of compression 

 than those with less complicated structures. This is exactly what is 

 found. For example, nitrogen, in the region in which interference be- 

 tween the molecules begins and the ideal gas laws lose their validity, 

 loses its compressibility much more rapidly than does helium, because 

 tlie molecules of nitrogen are larger and are more quickly brought into 

 contact. At still higher pressures, however, when the vacant spaces be- 

 tween molecules have been largely squeezed out, nitrogen becomes more 

 compressible than helium, because the molecule of nitrogen is larger 

 and more complex than that of helium, and therefore possesses the 

 possibility of undergoing a greater degree of compression. 



It is, of course, to be understood that there is no sharp dividing line 

 between these three mechanisms by which a substance responds to an 

 external pressure by losing volume ; at any instant all three mechanisms 

 are present together. It is the relative importance of the three mech- 

 anisms which changes with increasing pressure. It has already been 

 suggested that at still higher pressures the atoms themselves may begin 

 to break. As a result of its complex structure an atom may conceiv- 

 ably break in many ways. It is quite possible that what happens may 

 not be anything like as catastrophic and irreversible as the change we 

 ordinarily associate with breaking; it may merely be a rearrangement 

 of the electrons in their orbits, and there is no reason why such a rear- 

 rangement should not be reversible. At pressures higher than those 

 yet reached in the laboratory, amounting perhaps to several millions of 

 atmospheres, we may expect all sorts of detailed changes of this kind to 

 be produced in atoms. In the laboratory, two instances have already 

 been found of an inner rearrangement presumably due to pressure. 

 The element cesium, which is the most compressible of the metals, 

 undergoes an abrupt change of volume at 45,000 atmospheres. This 

 change is large — 17 percent — and there seems to be no explanation for 

 it in terms of the ordinary lattice structure of the metal, because it is 

 highly probable that below 45,000 atmospheres the lattice is already 

 in the close-packed, face-centered cubic arrangement. It would seem 

 that pressure could not make a more closely packed arrangement than 

 one which is already close-packed. The explanation seems to be that 

 a rearrangement of the electronic orbits within the atoms is brought 

 about by pressure. The details have been worked out in a recent paper 

 by Sternheimer ; he shows that an electronic transition from a Qs zone 

 to a 5d zone exactly accounts for all the experimental facts. Metallic 

 cerium is doubtless a similar case ; it shows an abrupt volume change 

 at 7,000 atmospheres, of the same order of magnitude as that which 

 occurs in cesium. In the case of cerium it has been shown by ingen- 

 ious experiments at the University of Chicago that both above and 



