This is based on Graham's law, which states that the rate of effusion of a gas through a porous barrier is inversely proportional to the square root of the gas's molecular mass. In a container with a porous barrier containing a mixture of two gases, the lighter molecules will pass out of the container more rapidly than the heavier molecules. The gas leaving the container is slightly enriched in the lighter molecules, while the residual gas is slightly depleted. Simon's team conducted experiments with copper gauze as the barrier. Because uranium hexafluoride, the only known gas containing uranium, was both scarce and difficult to handle, a mixture of carbon dioxide and water vapour was used to test it.