September 8, 1898J 



NA TURE 



445 



by Prof. T. Preston in Dublin, and by Prof. Michelson and Dr. 

 Ames and others in America. 



It appears that the different lines in the spectrum are dif- 

 ferently affected, some of them being tripled with different 

 grades of relative intensity, some doubled, some quadrupled, 

 some sextupled, and some left unchanged. Even the two com- 

 ponents of the D lines are not similarly influenced. Moreover, 

 whereas the polarisation is usually such as to indicate that 

 motions of a negative ion or electron constitute the source of 

 light, a few lines are stated by the observers at Baltimore, who 

 used what they call the " small " grating of five inches width 

 ruled with 65,000 lines, to be polarised in the reverse way. 



Further prosecution of these researches must lead to deeper 

 insight into molecular processes and the mode in which they 

 affect the ether ; indeed already valuable theoretic views have 

 been promulgated by H. A. Lorenz, J. Larmor, and G. F. 

 Fitzgerald, on the lines of the radiation theory of Dr. John- 

 stone Stoney ; and the connection of the new phenomena with 

 the old magnetic rotation of Faraday is under discussion. It is 

 interesting to note that Faraday and a number of more recent 

 experimenters were led by theoretic considerations to look for 

 some such effect : and though the inadequate means at their 

 d isposal did not lead to success, nevertheless a first dim glimpse 

 of the phenomenon was obtained by M. Fievez, of the Royal 

 Observatory at Brussels, in 1885. 



It would be improper to pass without at least brief mention 

 the remarkable series of theoretic papers by Dr. J. Larmor, 

 published by the Royal Society, on the relationship between 

 ether and matter. By the time these researches become generally 

 intelligible they may be found to constitute a considerable step 

 towards the further mathematical analysis and interpretation of 

 the physical universe on the lines initiated by Newton. 



In the mechanical construction of Rontgen ray tubes I can 

 record a few advances : the most successful being the adoption 

 of Prof. Silvanus P. Thompson's suggestion of using for the 

 anti-kathode a metal of high atomic weight. Osmium and 

 iridium have been used with advantage, and osmium anti-kathode 

 tubes are now a regular article of manufacture. As long ago as 

 June 1896, X-ray tubes with metallic uranium anti-kathodes were 

 made in my own laboratory, and were found to work better than 

 those with platinum. The difficulty of procuring metallic 

 uranium prevented these e.\periments from being continued. 

 Thorium anti-kathodes have also been tried. 



Rontgen has drawn fresh attention to a fact very early ob- 

 served by English experimenters — that of the non-homogeneity 

 of the rays and the dependence of their penetrating power on 

 the degree of vacuum ; rays generated in high vacua have more 

 penetrative power than when the vacuum is less high. These 

 facts are familiar to all who have exhausted focus tubes on their 

 own pumps. Rontgen suggests a convenient phraseology ; he 

 calls a low vacuum tube, which does not emit the highly pene- 

 trating rays, a " soft " tube, and a tube in which the exhaustion 

 has been pushed to an extreme degree, in which highly penetrat- 

 ing rays predominate, a "hard" tube. Using a "hard" tube 

 he took a photograph of a double-barrelled rifle, and showed 

 not only the leaden bullets within the steel barrels but even the 

 wads and the charges. 



Benoit has re-examined the alleged relation between density 

 and opacity to the rays, and finds certain discrepancies. Thus, 

 the opacity of equal thicknesses of palladium and platinum are 

 nearly equal whilst their densities and atomic weights are very 

 different, those of palladium being about half those of platinum. 



At the last meeting of the British Association visitors saw — 

 at the McGill University — Profs. Cox and Callendar's apparatus 

 for measuring the velocity of Rontgen rays. They found it to 

 be certainly greater than 200 kilometres per second. Majorana 

 has made an independent determination, and finds the velocity 

 •to be 600 kilometres per second with an inferior limit certainly 

 of not less than 150 kilometres per second. It may be re- 

 membered that J. J. Thomson has found for kathode rays a 

 Nelocity of more than 10,000 kilometres per second, and it is 



tremely unlikely that the velocity of Rontgen rays will prove 



■ be less. 



Trowbridge has verified the fact, previously announced by 

 I'rof. S. P. Thompson, that fluor-spar, which by prolonged 

 heating has lost its power of luminescing when re-heated, regains 

 the power of ihermo-luminescence when exposed to Rontgen 

 rays. He finds that this restoration is also effected by exposure 

 to the electric glow discharge, but not by exposure to ultra- 

 violet light. The difference is suggestive. 



As for the action of Rontgen rays on bacteria, 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 

 sensitive to the rays is confirmed by Dorn and by Rontgen 

 himself. 



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

 confirmed by the fact ascertained 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 comprehension of the Rontgen rays had 

 been propounded before the rays were discovered. At the 

 Liverpool m.eeting 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 explan- 

 ation 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 experiments of Dr. L. Graetz. The details of this 

 theory are in process of elaboration by Prof. 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 kathode rays ; it becomes more and more clear that 

 kathode rays consist of electrified atoms or ions in rapid pro- 

 gressive motion. My idea of a fourth state of matter, pro- 

 pounded in 1 881 {Phil. Trails.^ Part 2, 1881, pp. 433-4), and 

 at first opposed at home and abroad, is now becoming accepted. 

 It is supported by Prof. J. J. Thomson {Phil. Mag., October 

 1897, p. 312) : Dr. Larmor's theory {Phil. Mag., December 

 1897, p. 506) likewise involves the idea of an ionic substratum 

 of matter ; the view is also confirmed by Zeeman's phenomenon. 

 In Germany — where the term kathode 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 favour 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 the aid of my own discovery nearly twenty 

 years ago, that their path is curved in a magnetic field, and that 

 they produce phosphorescence where they impinge on an 

 obstacle. The two unknown quantities, the charge and the 

 speed of each atom, are measurable from the amount of curv- 

 ature and by means of one other independent experiment. 



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 the violent collisions 

 due to their impact on a massive target placed in their path, 

 give rise to the interesting kind of extremely high frequency 

 radiation discovered by Rontgen. It has, indeed, for some time 

 been known that whereas a charged body in motion constitutes 

 an electric current, the sudden stoppage, or any violent accelera- 

 tion of such a body, must cause an alternating electric dis- 

 turbance, which, though so rapidly decaying in intensity as to 

 be practically "dead beat," yet must give rise to an ethereal 

 wave or pulse travelling with the speed of light, but of a length 

 comparable to the size of the body whose sudden change of 

 motion caused the disturbance. The emission of a high-pitched 

 musical sound from the jolting of a dustman's cart (with a spring 

 bell hung on it) has been suggested as an illustration of the way 

 in which the molecules of any solid not at absolute zero may 

 possibly emit such rays. 



If the target on to which the electrically-charged atoms 

 impinge is so constituted that some of its minute parts can 

 thereby be set into rhythmical vibration, the energy thus 

 absorbed reappears in the form of light, and the body is said to. 

 phosphoresce. The efficient action of the phosphorescent target 

 appears to depend as much on its physical and molecular as on 

 its chemical constitution. The best known phosphori belong to 

 certain well-defined classes, such as the sulphides of the alkaline- 

 earthy metals, and some of the so-called rare earths ; but the 



NO. 1506, VOL. 58] 



