158 CATHODE RAYS. 



The physicist, liowever, who did more than any one else to direct 

 attention to these rays was Mr. 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. 



Eecently 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 Eontgen rays. 



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. In the first place, when these rays fall on a substance 

 they produce changes physical or chemical in the nature of the sub- 

 stance. 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. 

 Goldstein found that these, when exposed to the cathode rays, changed 

 color, the change, according to E. Wiedemann and Ebert, being due to 

 the formation of a subchloride. 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 expo- 

 sure to the cathode rays. But though it is only in comparatively few 

 cases that the change produced by the cathode rays shows itself in 

 such a conspicuous v^ay as by a change of color, there is a much more 

 widely spread x^henomenon, which shows the permanence of the eftect 

 produced by the impact of these rays. This is the phenomenon called 

 by its discoverer. Prof. E. Wiedemann, thermoluminescence. 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. When, however, we heat the sub- 

 stance, 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. The substance retains this property 

 for months after the exposure to the rays has ceased. The phenom- 

 enon of thermoluminescence is especially marked in bodies which are 

 called by Van t'Hoff solid solutions. These are formed when two salts, 

 one greatly in excess of the other, are simultanoously precipitated from 

 a solution. Under these circumstances the connection between the 

 salts seems of a more intimate character than that existing in a mechan- 

 ical mixture. I have here a solid solution of CaSo4 with trace of 

 MnSo^, and you will see that after exposure to the cathode rays it 

 becomes luminous when heated. Another proof of the alteration pro- 

 duced by these rays is the fact, discovered by Crookes, that after glass 

 has been exposed for a long time to the impact of these rays, the 

 intensity of its phosj^horescence is less than when the rays first began 

 to fall upon it. This alteration lasts for a long time, certainly for 

 months, and Mr. Crookes has shown that it is able to survive the heat- 



