November 1, 1898.] 



KNOWLEDGE, 



249 



certainly a disadvantage, especially when a difficult radio- 

 graph, requiring a lengthy exposure, ia being taken. 

 While, therefore, taking advantage of Mr. Swinton's very 

 useful principle of varying the distance between anode 

 and cathode. Dr. Dawson Turner, in conjunction with the 

 writer, has reversed the arrangement by making the 

 cathode moveable and keeping the anode fixed ; and has 

 added a further modification in that the cathode is adjusted 

 by magnetic means, so that movement may be made easily, 

 without disturbing the tube at all, while it is in any 

 desired position. If the tube is constructed so that the 

 cathode, in its movement slides in and out of the side 

 annex blown in the bulb, so as to keep it in proximity to 

 the glass throughout its movement, it is found that the 

 latter has a greater influence upon the resistance of the 

 tube than mere movement to and fro when the cathode is 

 quite out into the bulb space, and affects it in the reverse 

 way ; that is, the nearer the cathode is to the anode the 

 lower the resistance, and this increases as the cathode is 

 gradually drawn back inside the annex. The above modifi- 

 cations for the variation in the penetration in the tube are 

 certainly an advance over the older, uncertain methods of 

 potash tubes, &c. 



Taking advantage of the fact, first suggested by Prof. 

 S. P. Thompson, that the higher the atomic weight of 

 the anode the higher the penetration of the tube, a further 

 advance has been made by Dr. Mackenzie Davidson, in 

 the use of osmium as an anode. The scarcity of the 

 metal, however, is the one great drawback to its use. 



Little has to be said in regard to the fluorescent screen. 

 Potassium-platino-cyanide and barium-platino-cyanide are 

 almost the only salts used, as nothing has yet been found 

 to approach them in efliciency. The latter is preferred on 

 account of the ease in working it. The salt now obtain- 

 able is by far purer than that of two years ago, con- 

 sequently screens are made with much more uniform and 

 brilliant surfaces. 



Mention must be made, however, of the great assistance 

 screens offer in shortening the time of exposure, especially 

 of the more inaccessible parts of the body. The salts 

 fluorescing green, however, such as barium-platinocyanide, 

 are by no means so active as calcium tungstate, which 

 fluoresces blue, and which has, therefore, a greater photo- 

 graphic activity. Placing the fluorescing surface of the 

 screen in contact with the film of the plate, the exposure 

 is, in some cases, reduced to one-fifth of the time otherwise 

 required. Special plates, however, give the best results, 

 and it is difiicuU to eliminate the granulation of the 

 screen. 



What, now, is the mechanism producing Riintgen rays '? 

 Do they consist of molecular streams, or are they of the 

 nature of vibrations — transverse or longitudinal ? Here 

 we are confronted with a host of hypotheses and theories 

 that would demand much more space than is here possible, 

 to discuss adequately. 



Experiments by Eontgen, Battelli, and others, have 

 tended to show that Eontgen and cathode rays are of the 

 same nature, but that the former constitute only part of 

 the latter. The clear distinction, however, between actual 

 similarity is expressed by the absolute non-deviation of 

 Eontgen rays in a magnetic field, while this phenomenon 

 is a strong characteristic of cathode rays. The numerous 

 researches by Swinton and others seem to place beyond 

 doubt the molecular nature of cathode rays, and to prove 

 that they consist of electrified atoms or ions in rapid pro- 

 gressive motion, while the general opinion of physicists 

 seems to be settling towards a wave or ether theory for the 

 Eontgen rays. The difficulty of formulating a perfectly 

 satisfactory theory is great, however, when one has to 



contend with the fact that there is no direct proof of 

 reflection, refraction, or even polarization of the rays. If 

 polarization could be proved it would simplify matters, as 

 it would show the vibrations to be transversal. The three 

 principal hypotheses under discussion at the present time 

 are : — Firstly, the ultra-corpuscular theory, by Prof. J. J. 

 Thomson ; secondly, that the rays are transverse ether 

 waves, and of such excessively short wave lengths that 

 they are an extreme case of ultra violet light ; thirdly, the 

 hypothesis of Sir O. Stokes, that they consist of transverse 

 waves in the same manner as light waves, but that they 

 differ from the latter in that they do not form regular trains 

 of wavelets — half a milUon or more, on the average, in 

 each train— but are solitary waves, each " train" consisting 

 of but one or two wavelets at the most. 



The first of these theories is truly a startling one, for it 

 assumes that the atoms of ordinary matter can be pul- 

 verized into stiU finer particles, and that even solid 

 bodies may be penetrated by the flight of such sub- 

 atoms travelling with enormous velocity. It also opens 

 up the question of the divisibility of the atom, which, to 

 say the least of it, is an amazing one to face. 



Stokes' theory amounts to this ; — That cathode rays 

 consist of negatively charged missiles, shot in showers like 

 hedge-firing, from the negative electrode against a target 

 (the anti-cathode), which receives and suddenly arrests 

 them ; and that the Eiiutgen rays are due to the indepen- 

 dent pulses propagated through the ether when the 

 advances of their negative charges are thus abruptly 

 stopped or altered. The radiation from the target reaches 

 the object which is being skiagraphed as an undulation 

 consisting of irregular pulses. 



This view has been advanced by Johnston Stoney in 

 analysing these irregular undulations and resolving them 

 into trains of waves of different wave-lengths, among which 

 waves of short wave-length are abundant if the hedge- 

 firing has been sulliciently violent and irregular. The 

 object will then be opaque to the longer waves but trans- 

 parent to the short ones, and the Eontgen effects follow. 

 This explanation tends to bring Stokes' theory into 

 agreement with the above theory of Sagnac and others, 

 that the rays are of the nature of light waves, but with 

 excessively short, ultra-violet wave-lengths. 



HANDICRAFT IN THE LABORATORY.* 



GIVEN — a piece of lead glass tubing, two inches in 

 diameter ; to hermetically seal in the tube a 

 specimen of chlorine gas in order to display the 

 yellowish green colour ; how many chemists or 

 physicists could do it so as to make the specimen 

 presentable on a lecture table or for exhibition in a museum? 

 Such a task demands more skill in craftsmanship than most 

 students are ever able to command. Either traces of the 

 reduced lead, through imperfect management of the 

 oxidizing and reducing flames, will be left in the glass to 

 mar the specimen, or lack of symmetry will appear in the 

 two ends of the sealed tube due to inefficient control of the 

 various forces which tend to alter the shape of the glass 

 while in the plastic condition. A somewhat simpler 

 (though by no means easy) task in the manipulation of the 

 blowpipe is to seal up a specimen of sodium or potassium 

 in vacuo, or in an atmosphere free from oxygen, so as 

 to exhibit and retain the brilhant silvery lustre of these 

 metals when their inordinate propensity for appropriating 

 the vital element is thus held in check. True it is that some 



* '• Glass Blowing and Working." By Thomas Bolaa, P.C.S., F.I.C., 

 etc. (Dawbarn &. Ward.) Illustrated. 2s. net. 



