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. A container wherein the enrichment process takes place through gaseous diffusion is called a diffuser. Gaseous diffusion had been used to separate isotopes before. Francis William Aston had used it to partially separate isotopes of neon in 1931, and Gustav Ludwig Hertz had improved on the method to almost completely separate neon by running it through a series of stages. In the United States, William D. Harkins had used it to separate chlorine. Kistiakowsky was familiar with the work of Charles G. Maier at the Bureau of Mines, who had also used the process to separate gases. Uranium hexafluoride () was the only known compound of uranium sufficiently volatile to be used in the gaseous diffusion process. Before this could be done, the Special Alloyed Materials (SAM) Laboratories at Columbia University and the Kellex Corporation had to overcome formidable difficulties to develop a suitable barrier. Fortunately, fluorine consists of only a single isotope , so that the 1percent difference in molecular weights between and is due solely to the difference in weights of the uranium isotopes. For these reasons, was the only choice as a feedstock for the gaseous diffusion process. Uranium hexafluoride, a solid at room temperature, sublimes at at . Applying Graham's law to uranium hexafluoride: where: Rate1 is the rate of effusion of 235UF6. Rate2 is the rate of effusion of 238UF6. M1 is the molar mass of 235UF6 ≈ 235 + 6 × 19 = 349g·mol−1 M2 is the molar mass of 238UF6 ≈ 238 + 6 × 19 = 352g·mol−1 Uranium hexafluoride is a highly corrosive substance. It is an oxidant and a Lewis acid which is able to bind to fluoride. It reacts with water to form a solid compound, and is very difficult to handle on an industrial scale. Organization Booth, Dunning and von Grosse investigated the gaseous diffusion process. In 1941, they were joined by Francis G. Slack from Vanderbilt University and Willard F. Libby