Two tubes, one filled with bromine vapor and air, and the other with bromine gas and a small amount of liquid bromine. The tubes are cooled in a dry ice/alcohol bath. In the gas-air tube the bromine will solidify at the bottom of the tube. When it is removed from the cold bath, the gas will move up the tube until it is evenly dispersed. In the gas-liquid tube, the bromine will melt as the tube warms and the color will remain the same throughout the tube but darken as more of the bromine melts. In this tube it is difficult to determine which way the gas is moving. This demo demonstrates the dependence of diffusion on both temperature and density of gas.