l^OVEMBKK 4, 1898.] 



SCIENCE. 



605 



Trowbridge has verified the fact, pre- 

 viously announced by Professor S. P. 

 Thompson, that fluor-spar, which by pro- 

 longed heating has lost its power of lumi- 

 nescing when reheated, regains the power of 

 thermo-luminescence when exposed to Eont- 

 gen rays. He finds that this restoration is 

 also efieeted by exposure to the electric- 

 glow discharge, but not by exposure to the 

 ultra-violet light. The difference is sug- 

 gestive. 



As for the action of Rontgen rays on bac- 

 teria, often asserted and often denied, the 

 latest statement by Dr. H. Eieder, of 

 Munich, is to the elfect 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 sensitive to the rays is 

 confirmed by Dorn and by Eontgen himself. 



The essential wave-nature of the Eontgen 

 rays appear to be confirmed by the fact as- 

 certained by several of our great mathe- 

 matical 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 com- 

 prehension of the Eontgen rays had been 

 propounded before the rays had been dis- 

 covered. At the Liverpool meeting of the 

 British Association several speakers, headed 

 iby Sir George Stokes, expressed their con- 

 viction that the disturbed electric field 

 caused by the sudden stoppage of the mo- 

 tion of an electrically-charged atom yielded 

 the true explanation of the phenomena ex- 

 traneous to the Crookes high- vacuum tubes 

 — phenomena so excellently elaborated by 

 Lenard and by Eontgen. 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 polariza- 

 tion, an absence once more confirmed by 

 the careful experiments 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 Eontgen rays, 

 an opposite drift is apparent with respect to 

 the physical nature of the cathode rays ; 

 it beomes more and more clear that cathode 

 rays consist of electrified atoms or ions in 

 rapid progressive motion. My idea of a 

 fourth state of matter, propounded in 1881,* 

 and at first opposed at home and abroad, is 

 now becoming 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 

 Germany — where the term cathode ray was 

 invented almost as a protest against the 

 theory of molecular streams propounded by 

 me at the Sheffield meeting of the British 

 Association in 1879 — additional proofs have 

 been produced in favor of the doctrine that 

 the essential fact in the phenomenon is 

 electrified radiant matter. 



The speed of these molecular streams 

 has been approximately measured, chiefly 

 by aid of my own discovery nearly twenty 

 years ago, that their path is curved in a 

 magnetic field, and that they produce phos- 

 phorescence where they impinge on an ob- 

 stacle. The two unknown quantities, the 

 charge and the speed of each atom, are 

 measurable from the amount of curvature 

 and by means of one other independent ex- 

 periment. 



It cannot be said that a complete and 

 conclusive theory of these rays has yet 

 been formulated. It is generally accepted 

 that collisions among particles, especially 



*PMl. Trans., Part 2, 1881, pp. 433-434. 

 ■\PMl. Mag., October, 1897, p. 312. 

 tPhil. Mag., December, 1897, p. 506. 



