ADSORPTION IN VACUUM TUBES BY WILDER D. BANCROFT
It is a little hard to tell whether the change in the vacuum of a Crookes’ tube can be said to be due to the adsorption of the gas by the glass or not. There are some rather interesting phenomena which have been studied by Swinton,I who examined some tubes which had been in operation, nearly ten years before, for many hours “under extremely severe conditions, the alternating current employed having a volume of some 2 0 milli-amperes a t about 8000 volts pressure. “The condition of the glass walls of these tubes is now found to be as follows: When cleaned by rubbing they are quite transparent t o casual inspection, though careful examination with the microscope shows that the inner surfaces of those portions that had been subjected t o cathode ray bombardment are materially roughened. When these portions of the glass are heated strongly in a blow pipe, they immediately become clouded, the effect being due t o quantities of minute spherical bubbles in the glass, which may clearly be seen with a microscope. The bubbles vary to some extent in size, but on the average are about 0.01mm in diameter. They are usually packed closely together in a single layer and are always very near the side of the glass that formed the inner surface of the tube. On measuring the thickness of the glass with a micrometer gauge, dissolving the inner surface with hydrofluoric acid until the bubbles had just disappeared and then again measuring the thickness, it is found that, after making allowance for the size of the bubbles, the centres of these bubbles were about 0.122 mm from the inner surface of the glass. A similar estimate was obtained by grinding the inner surface of the glass until the bubbles just disap’pear and making micrometer measurements as before. It would, therefore, appear that the particles of gas must have been shot into the glass to about the depth stated. “In a typical piece of glass the number of bubbles per Proc. Roy. SOC., 79A, 134 (1907).
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square ’ centimeter of glass was found to be about 625,000, so that, allowing for the expansion of the gas on heating the glass up to redness, there was occluded about o.ooo113cc of gas at atmospheric pressure per square centimeter of glass, and as the amount of glass surface that was bombarded in each tube was about 400 cm2the total amount of gas a t atmospheric pressure occluded in each tube is nearly 0.05 cc. apart from any further amount that may have escaped from the glass in the heating of the latter. An interesting question arises as to whether the gas is merely mechanically mixed with the glass or whether there has been any chemical combination between the two. In the latter case, i t seemed unlikely that mere powdering of the glass would cause the gas to come out, while in the former case it seemed probable that powdering the glass would have this effect.” The experiment was tried of powdering the glass inside a vacuum tube and gas was given off which was found to be mainly hydrogen. ‘It would appear, therefore, that the gas occluded in soda-glass vacuum tubes exhausted in the ordinary manner from air is almost entirely hydrogen, no doubt due to electrolysis of water vapor condensed on the walls of the tube prior to exhaustion, the oxygen of which is adsorbed by oxidation of the aluminum electrodes.” As a further test the experiment was repeated with helium in the tube. On heating the glass, bubbles were obtained as before and helium was given off when the glass was crushed. “Seeing that helium does not combine with anything at ordinary temperatures, and seeing further that it could be extracted from the glass by the mere mechanical powdering of the latter, it would appear that the occlusion is due to the mechanical driving of the gas into t h e glass and not to any chemical combination.” The weak points in this argument are that the author does not consider the possibility of adsorption as something different from “mechanical entanglement,” and that it does not appear definitely whether the glass which was pulverized had previously been heated so as to cause the formation of visible
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bubbles. If it had been heated, the experiments prove nothing as to adsorption. In case the glass was not heated, as seems probable, we must consider what the author means by mechanical entanglement. It may be that the bubbles of gas were driven into the glass and that the glass flowed round behind them enclosing the bubbles. This would certainly be mechanical entanglement and it certainly would not be adsorption any more than one would speak of the air inside a sealed tube as being adsorbed. On this assumption the gas would be retained until the glass was heated or pulverized. On the other hand, it seems rather improbable that the glass would flow round the bubble in this way. If not, we must assume that the gas was shot into the glass, forming bubbles which still communicated with the inner part of the tube by fine pores. On this assumption the gas would actually be adsorbed in the pores. Since a capillary pore will adsorb more gas than an exposed surface, it is not surprising that the gas escaped when the glass was pulverized, quite apart from any question of heating or electrification which may occur while the glass is being pulverized. Until it is known whether or not the glass flowed round the bubbles and enclosed them, it is impossible t o say whether this is a case of mechanical entanglement or of adsorption. Unfortunately this did not occur t o Swinton and will have to be decided by somebody else. The general results of this paper are as follows : I . In a Crookes tube gas bubbles may be driven into the glass for a distance of about 0.1mm. 2 . This gas is set free when the glass is pulverized. 3. It is not known whether the glass flows round behind the bubbles cutting them off completely from the inner portion of the tube, or whether the holes made by the bubbles in passing into the glass do not close completely. 4. If the bubbles are sealed in the glass, this is merely a case of mechanical entanglement; but i f the bubbles are connected with the inner portion of the tube by fine pores, it is a case of adsorption. 5. Hydrogen and helium tubes behave alike. Cornetl University
