The occlusion of the residual gas by the glass walls of vacuum tubes

Users of Crookes’ tubes for Röntgen ray purposes have for long been aware that, apparently owing to a change in vacuum, the tubes become “harder” with use, and that it becomes necessary in the case of old tubes to heat the glass in order to get the discharge to pass. In a paper read before the Royal Society in April, 1899, the present writer pointed out that the vacuum actually increases in cathode ray tubes with use, even if repeated small doses of additional air be let in from time to time.