24 REPORT — 1898. 



Trowbridge has verified the fact, pre\ iously announced by Professor 

 iS. P. Thompson, that fluor-spar, which by jn-olonged heating has lost its 

 power of luminescing when re-heated, regains the power of thermo-lumi- 

 nescence when exposed to Rontgen rays. He finds that this restoration 

 is also effected by exposure to the electric glow discharge, but not by expos- 

 sure to ultra-violet light. The difference is suggestive. 



As for the action of Rontgen rays on bacteiia, often asserted and often 

 denied, the latest statement by Dr. H. Rieder, of Munich, is to the effect 

 that bacteria are killed by the discharge from ' hard ' tubes. "Whether 

 the observation will lead to results of pathologic importance remains 

 to be seen. The circumstance that the normal retina of the eye is 

 slightly sensiti\e to the rays is confirmed by Dorn and by Rontgen 

 himself. 



The essential wave-nature of the Rontgen rays appears to be con- 

 firmed by the fact ascertained by several of our great mathematical 

 physicists, that light of excessively short wave-length would be but 

 slightly absorbed by ordinary material media, and would not in the 

 ordinary sense be refracted at all. In fact a theoretic basis for a comj^re- 

 hension of the Rontgen rays had been propounded before the rays were 

 discovered. At the Liverpool meeting of the British Association, several 

 speakers, headed by Sir George Stokes, expressed their conviction that the 

 disturbed electric field caused by the sudden stoppage of the motion of an 

 electrically charged atom yielded the true explanation of the phenomena 

 extraneous to the Crookes high vacuum tubes — phenomena so excellently 

 elaborated by Lenard and by Rontgen. More recently, Sir George Stokes 

 has re-stated his 'pulse' theory, and fortified it with arguments which 

 have an important bearing on the whole theory of the refraction of light. 

 He still holds to their essentially transverse nature, in spite of the absence 

 of polarisation, an absence once more confirmed by the careful experi- 

 ments of Dr. L. Graetz. The details of this theory are in process of 

 elaboration by Professor J. J. Thomson. 



Meantime, while the general opinion of physicists seems to be settling 

 towards a wave or ether theory for the Rontgen rays, an opposite drift is 

 apparent with respect to the physical nature of the cathode rays ; it be- 

 comes more and more clear that cathode rays consist of electrified atoms 

 or ions in rapid progressive motion. INIy idea of a fourth state of matter, 

 propounded in 1881, • and at first opposed at home and abroad, is now be- 

 coming accepted. It is supported by Professor J. J. Thomson :- Dr. 

 Larmor's theory'' likewise involves the idea of an ionic substratum of 

 matter ; the view is also confirmed by Zeeman's phenomenon. In Ger- 

 many — where the term cathode ray was invented almost as a protest 

 against the theory of molecular streams propounded by me at the Sheflield 

 meeting of the British Association in 1879 — additional proofs have been 



' Phil. Trans., Part 2, 1881, pp. 433-4. 



■-• PMl. Mag., October 1897, p. 312. ' Ibid., December 1897, p. 506. 



