The adsorption of oxygen on incandescent tungsten, at gas pressures of the order of 10-6 mm Hg is known to result in a layer of extreme stability, only capable of removal by simple evaporation at temperatures above 2000° Abs or by formation of the comparatively volatile WO3 when attacked by excess oxygen. Langmuir and Villars estimated the heat of adsorption of the oxygen as 162,000 calories per gram atom, which is so much greater than the heat of dissociation as to make extremely plausible the view that this stable layer is monatomic. Langmuir and Villars's calculation, however, was based on an indirect method, in which the adsorbed oxygen acted as a cement in attaching a layer of caesium to the underlying metal; the effect of the caesium on the thermionic emission from the tungsten in turn served as an indicator of the presence of the oxygen. A cathode-ray oscillographic method of measuring transient gas phenomena at a thermionic surface, developed in Part I, offers a simple means of investigating directly the evaporation of this oxygen layer without having to rely on the unknown variables involved in its effect upon a third substance such as caesium. The method consists essentially of photographing the track of the oscillograph spot as it responds to the variations in potential across a high resistance carrying the thermionic current from a pure filament, before, during, and after the latter's exposure to an adsorbable gas. Since an oxygen layer greatly reduces the normal electron emission from tungsten, the oscillograph traces with a delay of less than 10-6 seconds any changes occurring in the oxygen covering of the filament surface. The special methods devised for giving accurately repeatable time traverse to the oscillograph spot are described, with an analysis of the time lags inherent in the study of high-velocity reaction, in the paper referred to. Sample photographs of the thermionic changes accompanying evaporation of oxygen and of oxide are also there included. The contrasting stabilities appear in a volatilization of oxide within a second at a temperature low enough for the oxygen itself to be retained on the metal for many minutes.
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Johnson et al. (1935) studied this question.