This process was based on the fact that when a mixture of two gases passes through a temperature gradient, the heavier gas tends to concentrate at the cold end and the lighter gas at the warm end. That this can be used as a means of isotope separation was first demonstrated by Klaus Clusius and Gerhard Dickel in Germany in 1938, who used it to separate isotopes of neon. They used an apparatus called a "column", consisting of a vertical tube with a hot wire down the centre. The advantage of the technique was that it was simple in design and there were no moving parts. But it could take months to reach equilibrium, required a lot of energy, and needed high temperatures that could cause a problem with the uranium hexafluoride.