Urey and Murphy calculated from the Debye model that the heavy isotope would have a slightly higher boiling point than the light one. By carefully warming liquid hydrogen, 5 litres of liquid hydrogen could be distilled to 1 millilitre, which would be enriched in the heavy isotope by 100 to 200 times. To obtain five litres of liquid hydrogen, they traveled to the cryogenics laboratory at the National Bureau of Standards in Washington, D.C., where they obtained the help of Ferdinand Brickwedde, whom Urey had known at Johns Hopkins.