SANDSTONE TORTUOSITY. In conventional nuclear magnetic resonance (NMR) imaging, a liquid is the working substance. For example, the hydrogen nuclei in watery living tissue are weakly oriented by a powerful magnet, and then these nuclei signal their positions by emitting radio waves. By contrast, gas-phase NMR imaging has been difficult because of the low density of gases, which yields only a weak NMR signal. Recently, however, practical NMR imaging has been realized for noble-gas atoms by strongly orienting the nuclei (with polarized laser light) outside the sample and then injecting them into, say, the lungs, where they rapidly diffuse into the deepest of alleyways, providing data that can't be collected in any other way. In a new extension of gas-phase NMR to the study of porous materials such as oil-bearing sandstone and carbonate rocks, the aim right now is not so much to provide images (the rapid diffusion of the gas atoms limits the spatial resolution, as one would expect for a moving target, to about one millimeter) as it is to characterize internal topology. Ronald Walsworth (617-495-7274, rwalsworth@cfa.harvard.edu) and his colleagues at the Harvard-Smithsonian Center for Astrophysics and Schlumberger-Doll inject xenon atoms into various porous rock samples filled with countless pores and connections, which affect the rate of gas diffusion and flow in the porous solid. They determine such things as the pore surface-area-to-volume ratio and a property called 'tortuosity,' which is an indication of how the structure of the porous medium restricts the flow of gases or liquids through the material. In this sense, tortuosity is to fluid flow what the structure of a wire (cross-section, length, etc.) is to the flow of electricity. Noble gases may be handier to use than liquids in NMR studies of rocks and other porous materials since the gas can flow further and faster through the pores without losing its orientation. (R.W. Mair et al., Physical Review Letters, 18 October 1999; science writers can obtain copies of the text from the Physics News Select Articles web site.