378 



NA TURE 



[February 20, 1896 



the rays, however, occurs when the potential of the plate does 

 not exceed that of the tin-plate cover by more than 3 or 4 volts, 

 and I have not yet met with any phenomena which suggest that 

 there is a lower limit of potential difference below which leakage 

 does not take place. 



" This leakage differs from that produced by ultra-violet light, 

 the laws of which have been unravelled by Elster and Geitel, in 

 several essential features, in the first place ultra-violet light only 

 discharges a negative charge, while the Rontgen rays discharge 

 both positive and negative. Again, the effect of ultra-violet 

 light is only considerable when the electrified body is a strongly 

 electro-positive metal with a clean surface. The effects of the 

 Rontgen rays are, on the other hand, very marked whatever the 

 metal, and take place when the electrified plate is surrounded 

 by solid or liquid insulators as well as when surrounded by air. 

 I have embedded the plate in solid paraffin wax, in solid sul- 

 phur, placed it inside a lump of ebonite, wedged it in between 

 pieces of mica, and immersed in a bath of paraffin oil ; in 

 each of these cases, though the insulation was practically 

 perfect when the insulator was not traversed by the Rontgen 

 rays, and the potential of the plate differed from that of the 

 metal covering of the box by from 10 to 15 volts, yet, as soon as 

 the Rontgen rays passed through the insulator, the charge of 

 the metal plate leaked away. I have found that the electricity 

 leaks from the plate even when the space between it and the 

 nearest conductors connected to earth is entirely filled with solid 

 paraffin ; hence we conclude that when the Rontgen rays pass 

 through a dielectric they make it during the time of their passage 

 a conductor of electricity, or that all substances 7vheit transmit- 

 ting these rays are conductors of electricity. The passage of 

 these rays through a substance seems thus to be accompanied by 

 a splitting up of its molecules, which enables electricity to pass 

 through it by a process resembling that by which a current 

 passes through an electrolyte. By using a block of solid 

 paraffin in which two pairs of electrodes were embedded, the 

 line joining one pair being parallel, that joining the other pair 

 perpendicular, to the Rontgen rays, which were kept passing 

 through the block, I found that there is but little difference 

 between the rate of leakage along and perpendicular to the 

 rays." 



Prof. Thomson has investigated the question of longitudinal 

 vibrations in connection with the recent discoveries. In a 

 paper read before the Cambridge Philosophical Society on 

 January 27, he discussed the theory of longitudinal 

 waves from the point of view of the electro-magnetic theory 

 of light, and showed that on that theory longitudinal waves 

 can exist (i) in a medium containing moving charged ions; 

 (2) in any medium, provided the wave-length is so small as 

 to be compared with molecular dimensions, and the ether in the 

 medium is in motion. It was shown that it follows from the 

 equations of the electro-magnetic field that the ether is set in 

 motion in a varying electric field. These short waves would 

 not be refracted, but in this respect they do not differ from 

 transverse waves which on the electro-magnetic theory would 

 not be refracted if the wave-length were comparable with mole- 

 cular distances. The properties of the longitudinal waves were 

 developed in the paper. Prof. Thompson exhibited a number 

 of photographs which had been taken at the Cavendish Laboratory 

 by Prof. Rdntgen's method, and experiments made on the 

 Rontgen rays were described. In one of these experiments the 

 photographic plate was placed inside the vacuum tube so as to 

 intercept the rays between the kathode and the walls of the 

 tube ; in this case the plate was not affected, showing that the 

 fluorescence of the glass is necessary for the production of 

 these rays. Other experiments were made to see if they could 

 be excited by fluorescence without a kathode ; the ring discharge 

 was produced in bulbs, and caused a vivid phosphorescence ; a 

 plate protected by cardboard when exposed to the bulb for an 

 hour was not affected, nor was any greater effect produced when 

 the bulb was filled with a gas such as oxygen, which phosphoresces 

 under the discharge. It thus appears that both a kathode and a 

 phosphorescent substance are required for the production of 

 these rays, and that one without the other is inoperative. A 

 series of experiments were made by taking photographs through 

 tourmaline plates, (l) with their axes parallel, (2) with their 

 axes crossed ; it was hoped by this method to get some evidence 

 as to whether the rays were longitudinal or transverse. A con- 

 siderable number of photographs were taken in this way, but no 

 difference could be detected in the obstruction offered to the 

 rays by the tourmaline plates in the two cases. Another method 



NO. 1373, VOL. 53] 



of investigating the same question was described, based on 

 Elster and Geitel's discovery of the influence of the plane of 

 polarisation of light on its power to discharge electricity from a 

 metallic surface. The experiments, which were not concluded 

 until the day after the meeting of the Society, show that these 

 rays exert the most powerful effect in discharging electricity, 

 whether positive or negative, from an insulated electrified metal 

 plate exposed to their influence. A bulb separated from the 

 charged plate by a board three-quarters of an inch thick covered 

 with several layers of tinfoil exerted a most powerful effect, 

 and it was not until the thickness of the metal between the bulb 

 and the electrified plate was nearly quarter of an inch that the 

 effect ceased to be perceptible. The electrified plate is a much 

 more delicate detector of these rays than the photographic one, 

 and is more suitable when measurements are required. These 

 results, though by no means conclusive, are in favour of the 

 vibrations being longitudinal. 



We have already mentioned, in our abstracts of the Comptes 

 rendus, and in notes, the many papers which have been read 

 before the Paris Academy descriptive of developments of 

 Rontgen's work. In France, both the chemical and surgical 

 sides of the discovery are being studied. As recorded in our 

 issue of February 6 (p. 324), Prof. Lannelongue, assisted by 

 MM. Barthelemy and Oudin, have demonstrated to the 

 Academy of Sciences the applicability of the discovery 

 to surgery, and observations made by Prof. Lannelongue since 

 then bear out his conclusions that in diseases where there 

 is an actual loss of substance in the bone, or an abnormal 

 growth of bony tissue, photography by means of Rontgen's 

 rays will be a valuable aid to diagnosis. The current number 

 of the Comptes rendtts contains several papers dealing with the 

 chemical properties of the rays. M. Meslans has studied the 

 influence of the chemical nature of substances on their trans- 

 parency to the rays. He has found that the varieties of carbon 

 — diamonds, graphite, and charcoal— and their compounds are 

 easily traversed by Rontgen's rays, as also are compounds of hy- 

 drogen, oxygen, and nitrogen. Alkaloids are also transparent, but 

 sulphur, iodine, and silicon are opaque. M. Charles Henry has 

 found that by coating coins opaque to the rays with phosphores- 

 cent sulphide of zinc, photographic impressions of substances 

 beneath the coins can be obtained, metals coated in this manner 

 appearing to lose their opacity to the Rontgen rays. M. 

 Henry has also found that phosphorescent sulphide of zinc 

 emits, in addition to green light, a large number of the new 

 actinic rays. 



The properties of Lenard rays — that is, kathode rays which 

 have travelled outside the tube in which they had been pro- 

 duced — are also being investigated. A note in the Chemical 

 News states that Prof. Slaby, of Charlottenburg, has obtained 

 good photographs by means of Lenard rays. Dr. J. Joly has 

 also been successful in this direction of work. At the Dublin 

 University Experimental Science Association, on February 11, he 

 showed photographs of various objects taken after the manner of 

 Rontgen by the Lenard rays. One of a pair of spectacles 

 within its case showed well the transparency of wood and 

 cardboard and the comparative opacity of glass, and the 

 still higher opacity of the heavy metals. Experiments on 

 refraction of the rays have in the case of paraffin oil given 

 no evidence of refraction. This substance proved very trans- 

 parent. Experiments upon the reflection of the rays have, 

 however, yielded positive results. The rays were reflected 

 by a silvered copper mirror on to a sensitive plate. The 

 plate was in the geometrical shadow of a thick lead shield, the 

 rays passing through a slot in the shield and through thick mill- 

 board before reaching the mirror, which was inclined at an angle 

 of about 45" to the plate. Exposures of two hours gave 

 distinct photographic effects. Reflection from a concave mirror 

 placed behind the plate and facing the sensitive film, did not 

 give any trace of a focus, but a darkening of the full diameter of 

 the mirror, and rather more marked at the edge ; thus suggest- 

 ing irregular reflection of the rays. The experiment of Prof. 

 J. J. Thomson on the discharge of a charged electroscope by the 

 rays was also shown by Dr. Joly, and, at the conclusion of the 

 meeting, it was demonstrated that this effect was not obtained 

 at short distances from the tube, the electroscope then becoming 

 positively electrified. 



Several important letters on the subject of Rontgen rays have 

 come to us. Lord Blythswood communicates the following, in 

 continuation of his letter published last week :— 



" Since I last wrote to you, I, at the suggestion of Lord Kelvin, 



