jj thomson cathode ray experiment led to the discovery of

These are formed when two salts, one greatly in excess of the other, are simultaneously precipitated from a solution. The phenomenon of thermoluminescence is especially marked in bodies which are called by Van t’Hoff solid solutions.

The substance retains this property for months after the exposure to the rays has ceased. When, however, we heat the substance, we find that a change has taken place for the substance now, when heated, becomes luminous at a comparatively low temperature, one far below that of incandescence.

Professor Wiedemann finds that if bodies are exposed to the cathode rays for some time, when the bombardment stops the substance resumes to all appearance its original condition. This is the phenomenon called by its discoverer, Prof. But though it is only in comparatively few cases that the change produced by the cathode rays shows itself in such a conspicuous way as by a change of color, there is a much more widely spread phenomenon, which shows the permanence of the effect produced by the impact of these rays. Elster and Geitel have recently shown that these substances become photoelectric-i.e., acquire the power of discharging negative electricity under the action of light after exposure to the cathode rays. Wiedemann and Ebert, being due to the formation of a subchloride. Goldstein found that these, when exposed to the cathode rays, changed color, the change, according to E. In some cases this change is marked by a change in the color of the substance, as in the case of the chlorides of the alkaline metals. In the first place, when these rays fall on a substance they produce changes physical or chemical in the nature of the substance. I shall confine myself this evening to endeavoring to give an account of some of the more recent investigations which have been made on the cathode rays. Recently a great renewal of interest in these rays has taken place, owing to the remarkable properties possessed by an offsprings of theirs, for the cathode rays are the parents of the Röntgen rays. Crookes, whose experiments, by their beauty and importance, attracted the attention of all physicists to this subject, and who not only greatly increased our knowledge of the properties of the rays, but by his application of them to radiant matter spectroscopy has rendered them most important agents in chemical research. The physicist, however, who did more than any one else to direct attention to these rays was Mr. Goldstein seems to have been the first to advance the theory, which has attained a good deal of prevalence in Germany, that these cathode rays are transversal vibrations in the ether. If the cathode were replaced by a luminous disk of the same size, this disk would not cast a shadow of a small object placed near it, for though the object might intercept the rays which came out normally from the disk, yet enough light would be given out sideways from other parts of the disk to prevent the shadow being at all well marked. This was a very important observation, for it showed that the rays casting the shadow came in a definite direction from the cathode. This observation was extended by Goldstein, who found that a well marked, though not very sharply defined, shadow was cast by a small body placed near a cathode of considerable area.

The subject was next taken up by Plücker’s pupil, Hittorf, who greatly extended our knowledge of the subject, and to whom we owe the observation that a solid body placed between a pointed cathode and the walls of the tube cast a well defined shadow. Plücker ascribed these phosphorescent patches to currents of electricity which went from the cathode to the walls of the tube and then for some reason or other retraced their steps. Plücker, who had made a very minute study of the effect of a magnetic field on the ordinary discharge which stretches from one terminal to the other, distinguished the discharge which produced the green phosphorescence from the ordinary discharge by the difference in its behavior when in a magnetic field. He owed the opportunity to do this to his fellow-townsman Geissler, who first made such vacua attainable. Plücker was the first physicist to make experiments on the discharge through a tube in a state anything approaching what we should now call a high vacuum. The first observer to leave any record of what are now known as the cathode rays seems to have been Plücker, who in 1859 observed the now well-known green phosphorescence on the glass in the neighborhood of the negative electrode.

Jj thomson cathode ray experiment led to the discovery of