For many reasons, the phenomena accom panying the discharge of electricity through vacuums have been greatly studied by physicists.
A perfect vacuum would (according to received theories) be a perfect non-conductor of electric ity; and although such a vacuum is unattain able by experimental methods, vacuums have been prepared, through which the discharge of a powerful induction coil cannot be forced. At ordinary atmospheric pressure, the discharge from an electrical machine, or an induction coil, passes through a gas intermittently, and in the form of thread-like bunches of sparks re sembling miniature flashes of lightning. Sup pose, now, that electrodes are sealed into the two opposite ends of a vacuum tube, and that the tube is gradually exhausted by a'good air pump. The discharge, at first resembling loose threads gathered together at the ends, gradually loses this appearance as the exhaustion proceeds, and after a time the whole tube becomes filled with pale light; and when the pressure becomes reduced to about the 10,000th part of an atmosphere, the discharge assumes a stratified or striated appearance. In a vacuum tube of cylindrical form, with an internal electrode at each end in the form of a circular metal disc with its plane perpendicular to the length of the tube, we observe, as the pressure approaches the value here given, that there is a notable difference in the appearance of the tube at the two ends. The negative electrode is often covered with a soft, velvety glow, either wholly or in patches. Outside of this there is a dark space called UCrookes' dark space," or the °first dark space"; the width (or thickness) of this being roughly (but not accurately) propor tional to the reciprocal of the density of the gas in the tube. According to Puluj, in a •tube such as here described, the thickness of the Crookes' dark spaces, when the residual gas is air, and the pressure is reduced to the 13,000th of an atmosphere, is about one inch. Next beyond the Crookes' dark space comes a luminous space which is called the °negative glow"; and next after this there is usually (but not invariably) a second comparatively non luminous region called the °second negative dark space," or, sometimes, the °Faraday dark space.' Next after this comes a glow which is
called the °positive column,* and which reaches all the way to the positive electrode. It is in this part of the tube that the striations, referred to above, occur. They consist in a succession of disc-shaped luminosities, spaced at fairly uni form distances so long as the tube is of uni form diameter and separated by dark intervals; the discs being perpendicular to the axis of the tube. The stratifications are sometimes absent, and when they are present they often have an irregular motion of translation along the tube, this motion being sometimes in one direction and sometimes in the other; that is, sometimes toward the negative electrode and sometimes away from it. When the tube contains a mix ture of gases there appears to be a separate series of stratifications for each constituent gas. The motion of the stratifications is best ob served by noting the reflection of the tube in a rapidly-revolving mirror and it is often of such a character as to make the stratifications appear confused to the unaided eye or to obliterate them completely. When the tube has movable electrodes it is found that a shift of the posi tion of the negative electrode (that is, the cathode), causes a corresponding shift in the positions of the stratifications; these behaving, in this respect, as though they were rigidly connected with the cathode; but they are not at all affected by shifting the position of the pos itive electrode (or anode), except that since they do not extend beyond the anode, they are successively obliterated as the anode passes 'them. The phenomena of these vacuum dis charges are very complicated, and special works dealing with these matters must be consulted for a really adequate description of them. The vacuum tubes produced at the factory of Hein rich Geissler, at Bonn, Germany, were of such excellence that the expressions °Geissler tubes" and °vacuum tubes" could at one time be re garded as almost synonymous.