ONE OF THE MOST CONVECTIVE FLUIDS EVER OBSERVED IN THE LABORATORY has been created by researchers in France (Bernard Castaing, CNRS, hydrodyn@labs.polycnrs-gre.fr). Convection is the process in which heat is carried by parcels of moving fluid. For example, heating a pan of water causes the lowermost layer of fluid to expand and become less dense, making it rise and carry its heat upward. Convection is characterized by the Rayleigh number (Ra), the ratio of the buoyant force of rising fluid to the product of viscosity (resistance to motion in the fluid) and the thermal diffusivity (how quickly temperature differences in the fluid get smoothed out). Heating a container of cold helium gas (4-5 Kelvins in temperature) from the bottom, the researchers measured Ra numbers in excess of 10¹⁴, a record matched only by a group at Chicago (X.-Z. Wu et al., Phys. Rev. Lett., 30 April 1990). However, when Ra exceeded 10¹¹, the researchers identified for the first time a transition in the fluid layers near the container's boundaries, from a laminar (streamline) flow of heated material to the "ultimate" regime of convection (beyond which no new regime exists) in which the whole fluid becomes turbulent and consequently transports heat more efficiently. Rayleigh-Bernard (RB) convection, as it is known, is important in many natural processes, such as the movement of air around the globe. (X. Chavanne et al., Upcoming article in Physical Review Letters., tent. 27 Oct.)