Number 39 (Story #3), July 1, 1991 by Phillip F. Schewe and Ben Stein CLUSTERS OF ARGON ATOMS can coexist in solid and liquid phases over a range of temperatures for a given pressure, unlike ordinary bulk matter, in which phase coexistence is only observed at a single temperature (the critical point) for a certain pressure. Based on theoretical work by Stephen Berry at the University of Chicago, UCLA experimentalist Robert Whetten and others have actually made spectroscopic measurements of argon which show both liquid and solid characteristics. Neither molecular nor bulk in nature, clusters are sometimes considered to be a fifth state of matter, in addition to solid, liquid, gas, and plasma. (Mosaic, Spring 1991.)