74. CAUSE OF LUMINOSITY OF GAS OUTSIDE THE DISCHARGE TUBE.—At ordinary pressures, in the cases of hydrogen and air, as has been noted, the gas became luminous in the observing tube, the effect being, of course, the same as entering open air, represented in Fig. A, beginning of this chapter. In order to determine the luminosity at less pressures, the gas, of whichever kind, was enclosed in a rather long observing tube and only at rather high vacua did the bluish and sometimes reddish gaseous luminosity disappear. Upon grasping the tube with the hand or approaching any conductor connected to earth, of large capacity, the column stopped at that point so that the remainder of the tube, beyond the hand, measured from the discharge, was dark. The phosphorescence on the glass wall of the tube produced by the cathode rays was not influenced in any way by outside conductors, such as the hand. Cathode rays themselves were not stopped apparently by the hand, because the phosphorescent screen and glass, located beyond the hand, became luminous. He concluded, therefore, that the glowing of the gas had no close connection with the cathode rays. He proved this also by deflecting the cathode rays in the discharge tube from a certain space, and yet the gaseous luminosity remained. As an exception, the cathode rays sometimes appeared to be closely associated with the light column. He attributed the luminosity of the gas in general, at low pressures, not to the cathode rays, but directly to the electric current or some kind of electric force, § 11 and 14, which, as already remarked, permitted sparks to be drawn from the aluminum window and surrounding points. The negative glow light in Geissler tubes, § 30, is also to be regarded as gas illuminated by cathode rays. (Compare Hertz, _Wied. Ann._, XIX., p. 807, ’83.) Between that phenomenon and the glow observed here and attributed to irradiation, there exists a correspondence, inasmuch as in both cases the light disappears at high exhaustions, § 53, appears fainter and larger when the pressure increases, § 54, and then becomes brighter and smaller, § 54. But, whereas, the glow in the Geissler tube has become very bright and small at 0.5 mm. pressure, the gas in our experiment remains much darker up to 760 mm. pressure, and yet the illuminated spot is much larger. This difference cannot, therefore, be attributed to an inferior intensity of the rays here used. But it will be explained, § 76, as soon as we can show that at higher pressures cathode rays of a different kind are produced, which are much more strongly absorbed by gases than the rays investigated hitherto and produced at very low pressures. [Illustration: USE OF STOPS IN SCIAGRAPHY. (PERCH.) § 107., p. 101. By Leeds and Stokes. ]