A Substitute for the McLeod Gauge

Abstract

THE results of our experiments do not confirm Mr. Fleuss's statements (NATURE, July 5, p. 12). Even if the water vapour is always at a pressure less than its equilibrium vapour pressure at room temperature, the McLeod gauge reads low and usually inconsistently. The reason is, of course, to be found in the absorption of water by glass. Water vapour will condense on glass when its pressure is raised, even when the final pressure is no greater than 0.001 mm.; the amount that condenses increases with the time, so that the reading of the gauge depends on the rate at which the mercury is raised. All this is quite familiar to those who work with modern high vacuum apparatus; but Mr. Fleuss's statements, if unchallenged, might mislead some who are only beginning to acquire their experience.