Rate of Vaporisation and Vapour Pressure: A Method of Measuring the Specific Area of a Surface

LANGMUIR (Phys. Rev., 2, 329; 1913) has developed a method for the determination of the vapour pressure of metals from measurements of the rate of vaporisation of metallic filaments in vacuo. The vapour pressure is calculated from the Knudsen equation, where m is the rate of evaporation in vacuo, after making the assumption, which Langmuir supports with a considerable amount of experimental data, that the accommodation coefficient is unity. In this way he determined the vapour pressure at high temperatures, first of tungsten (loc. cit.) and later, with other workers, of platinum, molybdenum, silver, gold, copper, and nickel (Jones, Langmuir, and Mackay, ibid., 30, 201; 1927).