The Vapor Pressures of Metals; a New Experimental Method

Quasi-static method of measuring vapor pressures.---The vessel A containing the liquid and surrounded by a furnace for maintaining a uniform temperature, is connected by one tube C to a manometer M and a reservoir containing a neutral gas such as nitrogen at a suitable pressure, and by another tube B to an intermittent pump. Outside the furnace the two tubes are connected to opposite sides of a differential manometer D. Successive portions of the nitrogen are pumped off through B until the manometer D begins to show a permanent difference of pressure; then the reading of M is the vapor pressure desired. The action depends on the fact that when the pressure in M is less than the vapor pressure, nitrogen can get from C to B to equalize any difference of pressure caused by the pump, only by diffusion against the up-streaming vapor in C, and inter-diffusion in the case of a tube 3 to 4 mm in diameter is slow. A test of the method gave values for the vapor pressure of mercury 170^∘{} to 203^∘{}C only.04 mm greater on the average than those of Smith and Menzies.Vapor pressure of lead, 1118^∘{} to 1235^∘{}C was found to vary from 5.70 mm to 19.70 mm in good agreement with the equation log₁₀p(mm)=-10372/T-log₁₀T-11.35, which also fits the best results of Egerton at lower temperatures. The heat of vaporization of lead at its melting point is calculated to be 46,300 cal. The chemical constant comes out -1.40 which is close to the value -1.59 computed from the quantum theory of monatomic gases.