152 



NATURE 



[June 14, 1900 



emits rays which reproduce the same phenomena as the 

 rays emitted by radium and polonium, and are 100,000 

 times the intensity of ordinary uranium rays. Certain 

 thorium compounds are also radio-active, a property first 

 established in these by G. C. Schmidt and Mme. Curie, 

 and subsequently investigated by R. B. Owens and 

 Rutherford. 



Since this article was in the printer's hands a paper 

 by Sir W. Crookes on the radio-activity of uranium, read 

 before the Royal Society on May 10, has been received. 

 The author records an entire absence of radio-active 

 effects in all the barium minerals in his cabinet from 

 which uranium was absent, while pitch-blende and other 

 minerals containing uranium and thorium excited a 

 photographic plate. Arrangements were then made for 

 working up half a ton of pitch-blende, and the radio- 

 activity of the uranium salts was definitely traced to 

 the presence of a foreign body, which Sir W. Crookes 

 has christened for the time UrX {i.e. the unknown 

 quantity in uranium), following a fashion initiated by 

 Rontgen, and which has previously led to the introduc- 

 tion into our vocabulary of such terms as " Xd air" 

 (Italian "aria Xata" or ixata). We would suggest the 

 name " Crookesium " as a substitute. Whether uranium-X 

 is or is not identical with radium seems not fully decided, 

 but it appears to be distinct from polonium. It is now 

 proposed to try to separate the radio-active component 

 of thorium. 



Le Bon, who claims to have anticipated the Becquerel 

 rays in his " lumi^re noire," has expressed the opinion 

 that the properties attributed to radium and polonium 

 do'not prove the existence of new elements, and may be 

 accounted for by supposing the radio-active substances 

 to be mere allotropic modifications of bismuth and 

 barium. On this view there is no more fundamental 

 difference between the properties of radio-active and 

 ordinary barium than between phosphorescent and 

 ordinary sulphuret of lime. Giesel, of Brunswick, also 

 has adopted the terms "radio-active barium" and 

 "radio-active bismuth" in preference to "radium" and 

 "polonium." In support of the opposite view, Demargay 

 has proved that radium possesses a characteristic 

 spectrum, and M. and Mme. Curie find that the atomic 

 weight of radio-active barium chloride is greater than 

 that of ordinary chloride, amounting in one specimen 

 to as much as 146 as against 137. 



The pitch-blende used in the preparation of these sub- 

 stances is obtained from Joachimsthal, in Bohemia. 

 Under the direction of Giesel, working in co-operation 

 with Profs. Elster and Geitel, the firm of E. de Haen, of 

 List, near Hanover, have undertaken the preparation in 

 small quantities of radio-active barium emitting rays that 

 are unequalled in intensity, and have also placed on the 

 market cheaper by-products which also emit rays of 

 sufficient intensity to visibly excite a fluorescent screen. 

 The solid radioactive compounds of barium increase in 

 activity from the time of solidification, but do not reach 

 their maximum for more than a month. The barium 

 preparations are all luminescent, the chloride and bromide 

 especially so when dry. According to Giesel, the bis- 

 muth or polonium preparations lose their radio-activity 

 in a few weeks, and this property is also cited by Elster. 



The radio-activity of barium bromide is found by 

 Elster not to be destroyed by continuous heating for 

 twenty-four hours in vacuo. After cooling, the strength 

 is much reduced, but is restored after the lapse of a few 

 days to nearly the original intensity. 



Becquerel rays resemble Rontgen rays in their power 

 of " ionising " air, a property they possess to such a degree 

 as to discharge all conductors within a considerable dis- 

 tance of the radio-active substance. Their action on 

 electric sparks has been studied by Elster and Geitel. A 

 spark gap i cm. wide, consisting of a positive knob and a 

 negative disc, was exposed to the radiations from a barium 



NO. 1598, VOL. 62] 



preparation. The sparks or brushes were converted into 

 a violet glow-discharge, but the former discharge was re- 

 established on interposing a plate of lead. With discs 

 made of semi-conducting card the radium affected the 

 discharge at the distance of over i metre. According to 

 Elster, heating a small trace of a radio-active substance in 

 air in a Bunsen flame increases the electric dispersion of 

 the air of the room. 



Becquerel finds many bodies acquire the temporary 

 power of discharging conductors under the influence of 

 the rays, thus affording proof that these rays involve a 

 continuous emission of energy. The bodies do not, how- 

 ever, act on a photographic plate, and their activity is 

 lost on heating. This property is not assumed by the 

 double sulphate'of uranium and potassium. 



There appears at present no prospect of utilising 

 Becquerel rays as a substitute for Rontgen rays in 

 surgery. The difference of behaviour of the two kinds 

 of rays is well shown by two radiographs of the human 

 hand accompanying Dr. Walter's paper. In the one 

 taken with Rontgen rays the outlines of the bones are 

 remarkably clear and sharp ; in the other, taken with the 

 rays emitted by Giesel's most powerfully radio-active 

 preparations, a dark, ill-defined shadow of the outline ot 

 the hand is seen, but not a trace of the bones is visible. 

 This latter radiograph, which was taken with the rela- 

 tively short exposure of an hour, shows clearly the 

 shadows of a needle and of a coin that were placed under 

 the middle of the hand, proving that a certain proportion 

 of the rays had actually passed through the hand, but 

 without differentiating the bones from the rest. Experi- 

 ments undertaken by Walter to account for the hazy out- 

 line of the Becquerel radiographs point to the conclusion 

 that the Becquerel rays, when passing through substances 

 of small atomic weight, experience a far greater diffuse 

 scattering than Rontgen rays. Further, the secondary 

 radiations emitted by both light and heavy substances 

 under the influence of the Becquerel rays differ far less 

 from the incident rays in intensity and penetrability than 

 in the case of the secondary rays investigated by Sagnac 

 in connection with Rontgen rays. A further difference 

 lies in the far greater absorption of Becquerel rays by 

 specifically light substances, such as those forming the 

 flesh of the human hand. With the use. of a platino- 

 cyanide of barium screen, Walter observed the same 

 absence of all traces of bones as with photographic 

 methods, although the shadow of the hand was clearly 

 $een on the screen. 



The composite nature of Becquerel rays is suggested 

 by experiments on phosphorescence and selective absorp- 

 tion, as well as on magnetic deviation. Mme. Curie 

 has found that Becquerel rays are more easily absorbed 

 when they have already penetrated an absorbing layer 

 than when they have not. One aluminium disc absorbed 

 a certain proportion of the rays ; a second aluminium 

 disc absorbed an even greater proportion of the remainder. 

 According to the note on Mme. Curie's paper in Science 

 Abstracts, " this is due to the fact that the less penetrative 

 rays are absorbed in the first absorptive layers," but such a 

 view would more naturally lead one to expect that the pro- 

 portion of absorbed rays would be less at the second screen 

 than the first, instead of greater ; the phenomena can, 

 however, be accounted for by the hypothesis that the first 

 screen transforms the rays into secondary rays of lower 

 penetrating power. The existence of such secondary rays 

 has been supported by Villard, Meyer and Schweideler, 

 Dorn and others. Becquerel has, however, shown that in 

 the case of polonium rays from the Curies' preparations, 

 no secondary rays are emitted by aluminium. The phe- 

 nomenon of selective absorption has been studied by 

 Becquerel, who exposed various substances to the action 

 of radio-active barium chloride, including hexagonal 

 blende, platinocyanide of barium, diamond, and double 

 sulphate of uranium and potassium. The phosphorescence 



