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.