August 13. 1S96J 



NATURE 



)55 



quadrants of an electrometer. The disc was discharged by the 

 rays, and the transparency of the substance was measured by the 

 rate at whicli the spot of light from the electrometer needle 

 moved across the scale. Sheets of tinfoil were then substituted 

 for the glass, and the number — «, say— taken, such that the rate 

 of discharge of the disc was appro.ximately the same as with the 

 glass. The rate of discharge was accurately measured in the 

 two cases. The ratio of the rate of discharge with the gla.ss to 

 that with the n sheets of tinfoil gave a measure of their relative 

 transparency to Rontgen rays. The rays were then made to pass 

 through a number of sheets of tinfoil, and then through the 

 glass, and the rate of discharge measured. The glass was re- 

 moved, and the same n sheets of tinfoil as were formerly used 

 put in its place, and the discharge again measured. The ratio 

 of the rate of discharge in the latter two cases was a measure of 

 the relative transparency of the glass and the tinfoil .sheets to 

 Konlgen rays which had been already screened by passing 

 through tinfoil. If the Kcintgen rays were all of one kind, the 

 two ratios thus obtained should be equal, but a difference in the 

 ratios could only be explained by assuming that the rays were 

 not homogeneous, and that some were more readily absorbed by 

 the tinfoil, and others by the glass or other substance used. 

 \'aiious substances were tested against tinfoil in this manner. 

 With some there was no selective absorption, with others it was 

 very marked. Glass gave none, with mica and paraffin the 

 eficct was small, with fuchsine, eosine, and a number of other 

 substances the efi'ect was very inarked. A table, given in the 

 l»per, showed the results obtained with these and other sub- 

 stances. The author concluded that the Rontgen rays are of 

 diflcrent kinds, and that the substances given in the table diff^er 

 very much from tinfoil in their selective absorption. It is im- 

 portant to observe that these results were obtained with a 

 vacuum bulb which was working extremely well and discharging 

 the disc very rapidly. With another bulb, which was not nearly 

 so efficient, no evidence of selective absor|ition could be 0I3- 

 tained. The radiation frcjm this bulb was homogeneous as far as 

 could be determined by this experiment. With a third bulb, better 

 than the last, but not so good as the first, selective absorption 

 was obtained, although less marked than with the first bulb. 

 It seems, therefore, that as a tube becomes more efficient the 

 character of its radiation becomes less homogeneous. 



The effect of Rontgen rays in discharging electrified bodies 

 continues to form the subject of investigation of several Italian 

 physicists, whose conclusions may with interest be compared with 

 the results obtained by Prof. J. J. Thomson in this country. 

 Prof. Emilio \'illari (Alti della K. Aaademia dei Liiicci) 

 enunciates the following conclusions, in support of which an 

 elaborate series of experiments are fully described, (i) The 

 <lischargeof a conductor in air, when provoked by Rontgen rays, 

 lakes place by an electrical convection of the particles of air set 

 in action by the radiations. (2) The discharge is retarded if 

 the surface of the electrified conductor exposed to the air is 

 diminished by covering part of it with paraffin. (3) When 

 the conductor is covered all over with paraffin placed in 

 contact with it, the discharge stops almost immediately 

 after it has been started by the Rontgen rays. A little elec- 

 tricity conveyed by the .surrounding film of air charges the 

 liarafiin, and further discharge was prevented. (4) If the con- 

 ductor is surrounded by air enclosed within a tube of paraffin, 

 and subjected to Rontgen rays, the discharge at first takes place 

 fairly rapidly, but suUsequently proceeds extremely slowly. The 

 electricity carried off as usual by the air suddenly charges the 

 walls of the tube, and afterwards is dispersed with difficulty. 

 (5) The electricity dispersed by the body can be collected on a 

 tube of paraffin, as in the preceding case, or on an in.sulated 

 metal tube surrounding the discharged body. This collected 

 electricity can be observed with an electroscope, and it is, of 

 course, of the same kind as that of the body. (6) Metal tubes, 

 whether insulated or not, surrounding the electroscope, serve to 

 condense on it the charges imparted to them. They retard the 

 discharge produced by Rontgen rays, either on account of the 

 quantity of electricity accumulated by them, or owing to their 

 imperfect transparency to the rays. 



Prof Augusto Righi (Alii della A'. Aaad. dei Littcei) also 

 /////considers it " non proven" that any but a gaseous dielectric 

 becomes a conductor under the influence of Rontgen rays, thus 

 agreeing substantially with Prof, \illari. Prof Righi, how- 

 ever, has discovered a source of error in his previous experi- 

 ments, which, however, does not affect this result. If in front 

 of the aluminium window of a leaden box containing the 



NO. 1 398, VOL. 54] 



Crookes' tube, a large disc of leatl is placed, and the charged 

 body is situated in the geometrical shadow cast by the disc, it 

 might be supposed that no discharge would take place ; but 

 such is far from being the case, except when the leaden disc is 

 closely pressed against the window. 



In the succeeding number of the Alti del Lined this deflection 

 of Rontgen rays behind opaque bodies is discussed at con- 

 siderable length by Prof. E. \'illari, who claims to have recorded 

 the phenomenon as long ago as March last. Observations were 

 made on the discharge of an electroscope placed in different 

 portions of the shadow cast by a plate of lead, and, moreover, 

 photographic impressions were obtained upon a sensitive plate 

 placed just inside the geometrical umbra. Signer Villari con- 

 cludes, that in order to discharge an electroscope it is not 

 necessary that Rontgen rays should fall directly upon it. The 

 presence of air previously traversed by these rays is sufficient 

 to promote the discharge— a result in accordance with one o 

 Prof Rrintgen's original observations. 



In a subsequent paper in the same publication, Prof. Righ 

 proves that the discharge of electricity produced in air by 

 Rontgen rays takes place by convection along the lines of 

 electrostatic force. The experiments were made by means of an 

 insulated conducting sphere placed in the presence of a disc of 

 ebonite having its lower side covered by a metal armature. 

 Between the two a cross of ebonite was placed, and the con- 

 ductor and armature were oppositely charged. After exposing 

 the whole to the action of Rontgen rays for a few minutes, the 

 ebonite was dusted over with a mixture of powdered red-lead 

 and sulphur, when the .shadow of the cross appeared red on a 

 yellow background. In another experiment a cylindrical con- 

 ductor was substituted, and the shadow was produced by a strip 

 of ebonite placed parallel with its axis. The observed position of 

 the shadow agreed exactly with that calculated froin the form 

 of the lines of electrostatic force, which in this case were, of 

 course, coaxial circles. 



It .should be observed, however, that if the phenomenon were 

 one of conduction instead of convection, the discharge would 

 still follow the lines of force, just as in Prof. Righi's experiments. 

 The same indefatigable observer (Prof. Righi) also discusses 

 in the Comptes rendiis a paper by MM. Benoist and Hurmu- 

 zescu, who find that, " if Rontgen rays can develop an electric 

 charge . . . this effect does not exceed the order of magnitude 

 of the electromotive forces of contact." Prof. Righi finds that 

 the positive potential to which an insulated conductor is raised 

 when Rontgen rays fall on it, is precisely of that order of mag- 

 nitude. In the experiments of MM. Benoist and Hurmuzescu, 

 the electrometer and the conductor experimented on were 

 enclosed in an uninsulated metal case ; in Prof. Righi's experi- 

 ments the case was made to enclose the Crookes' tube, which 

 was placed at a considerable distance from the conductor, so as 

 to remove the latter as much as possible from the influence of all 

 conducting bodies. It appears probable that both dispositions 

 are equally good. 



Under the title " Raggi Catodici e Raggi-X " (Niiovo Cimenio), 

 Prof. Battelli and Dr. Garbasso give a continuation of their re- 

 searches bearing chiefly on the question of whether there is 

 really an essential difference between Rontgen rays and kathodic 

 rays. These writers are of opinion that the two kinds of 

 radiations do not differ from one another in any more essential 

 characteristics than those which enable us, for example, to dis- 

 tinguish two flames of different colour. 



In a further contribution to the Niioi'o Cimento, Prof Battelli 

 and Dr. Garbasso examine the resemblance between Rontgen 

 rays and ultra-violet light in their power of dispersing electric 

 charges. The experiments, which were made by employing 

 alternately a Crookes' tube and a volt.aic arc with the same 

 disposition of apparatus, lead to the conclusion that although 

 ultra-violet light acts on electrified bodies in the same manner 

 as Rontgen rays, the modification produced in the surrounding 

 air (in the case of ultra-violet light) is less pronounced and less 

 stable. 



.\n important point in connection with the debated nature of 

 Rontgen rays is the determination of their wave-length, which 

 has been successfully effected by Dr. L. Fomm, of Munich 

 (Sit^b. dcr Bayerischen Akademie, xxvi. ii.). -A.s these 

 rays show no measurable reflection or refraction, the only 

 way available was by difiVaction. The Rontgen rays emanating 

 from a large Hittorf tube were made to pass through a 

 brass slit 0-5 ram. in breadth, and, after being diffracted 

 by a second slit, were received on the photographic plate. The 



