420 Professor J. J. Thomson [April 30, 



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 offspring of theirs, 

 for the cathode rays are the parents of the Rontgen rays. 



I shall confine myself this evening to endeavouring 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 substance. In some cases this change is marked by a change 

 in the colour 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 colour, 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 photo- 

 electric, i.e. acquire the power of discharging negative electricity 

 under the action of light, after exposure to the cathode rays. But 

 thougb it is only in comparatively few cases that the change pro- 

 duced by the cathode rays shows itself in such a conspicuous way as 

 by a change of colour, there is a much more widely spread phenomenon 

 which shows the permanence of the effect produced by the impact of 

 these rays. This is the phenomenon called by its discoverer. Prof. 

 E. Wiedemann, thermoluminescence. Prof. 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 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 ; the substance retains this property for months 

 after the exposure to the rays has ceased. The phenomenon 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 simultaneously precipitated from 

 a solution. Under these circumstances the connection between the 

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

 mechanical mixture. I have here a solid solution of CaSo^ with 

 trace of MnSo^, and you will see that after exposure to the cathode 

 rays it becomes luminous when heated. Another proof of the altera- 

 tion produced 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 phosphorescence 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 heating up of the glass to allow of the remaking of the 

 bulb. I will now leave the chemical efiects produced by these rays, 

 and pass on to consider their behaviour when in a magnetic field. 



