MAXWELL'S DEMON MADE OF SAND. The second law of thermodynamics states that within a closed system heat cannot flow unassisted from a cold to a warm place. To ponder this issue, James Clerk Maxwell, one of the pioneers of statistical mechanics, posed this thought experiment: could not a clever microscopic creature, poised at a pinhole in a baffle dividing an insulated box into two equal chambers, sort molecules in such a way that the hotter (faster) molecules would be directed into one chamber while cooler (slower) molecules would be directed into the other. "Maxwell's demon," as the sorter came to be known, itself requires energy to operate, and so the segregation of hot from cold cannot really happen as advertised. And yet in an experiment conducted at the University of Essen in Germany in which agitated sand in a two-chamber vessel (the halves being connected by a hole) "hot," quickly moving sand migrated to one side while cool sand spontaneously condensed and congregated on the other side (see sketch at Physics News Graphics). Jens Eggers (011-49-201-183-3941, eggers@theo-phys.uni-essen.de) explains that, no, the second law is not violated in this case since although moving sand can be considered as a gas, individual grains can absorb heat and dissipate heat (that is, individual grains can gain temperature), unlike the ideal gas molecules described by Maxwell, whose "temperature" is a measurement of gas motion. Thus when sand grains start to congregate in one chamber (the segregation begins as an act of spontaneous symmetry breaking) more and more grains will partake of a growing ordered state consisting of grains falling to the bottom of the container (where the grains are denser there are more collisions and hence faster cooling, leading to more congregation, etc.), while the unaffiliated grains will tend to be on the other side, still in "gaseous" form. (Eggers, Physical Review Letters, 20 December; Select Articles.)