The Limiting Negative Pressure of Mercury in Pyrex Glass

Abstract

It is generally considered that mercury does not ``wet'' glass, as evidenced by the convex surface of the mercury meniscus in a manometer. If this were true under all conditions, the development of a negative pressure in a mercury column would be impossible. But pure mercury in a clean evacuated degassed tube shows a flat meniscus. In other words, when the tube is degassed, mercury does adhere to the glass wall. A U‐tube manometer of 5‐mm bore with one leg sealed off was evacuated and degassed at 500°C with a mercury‐vapor pump, and then filled with pure mercury by distillation. The vertical height of the mercury column from the meniscus in the open tube to the top of the closed leg was 52 cm. When the manometer was again evacuated, the mercury column remained suspended from the top of the tube, withstanding a negative pressure of two‐thirds of an atmosphere. In extending the measurements, short mercurial thermometers were first used. Each thermometer was cemented to the spinner of a variable‐speed motor, with the spin axis bisecting the distance from the end of the bulb to the meniscus. The maximum centrifugal stress on the free capillary column was consequently at the spin axis. Measurement of nine thermometers gave negative pressures ranging from 2 to 17 bars. Finally, freshly drawn capillary tubes with fine bores were evacuated, degassed by heating, sealed off and filled by breaking one end under pure mercury. The negative pressure which these mercury columns could withstand under centrifugal stress increased rapidly as the degassing improved. The highest negative pressure observed for mercury was 425 bars at 28°C. In this case, the furnace temperature during degassing was raised to the point where the capillary tube started to deform.