THE RADI(~>-ACTIYITY OF MATTER. 201 



being very ponetratino-. Uranium cniits principally the tirst kind, 

 polonium o-ives only the seconcU and radiiun oiv(\s all throe at onee. " 



I^et us now return to dcviable rays; the material th(M)ry of Sir 

 \\'illiam C'rookcs and Mr. .1. .1. Thomson can he appli(>d to them, and 

 the consequences can be veritied with th(> ureatest facility. In a uni- 

 form magnetic field the trajectories perpendicular to the iield are cir- 

 cumferential to the path'p which leads the rays to the point of emis- 

 sion. For an oblique (^mission making an angle with the field, the 

 trajectories are helices enveloping the cylinders with rays p sin a. 

 By })lacing on a horizontal photographic i)latc i)arall(d to the uniform 

 held, a small l»»;id box containing a few grains of racliferous barium 

 forming a source of wry small diameter, the rays wvo drawn (h)wn to 

 the plate, and excite it on one side alone; a bundle of sinqile rays 

 emitted in the planc^ normal to tlie ])late and paralhd to the field should 

 show theoretically an arc of an (dlipse of which the axes arc^ in the 

 proportion of 2 and tt. The accompanying photograpli (tig. 7) shows 

 these theoretical arcs, obtained 1)y revcn-sing the direction of the iield. 

 th(^ on(^ in air and the other in \acuo, on a photogra])hic plat(» envel- 

 ope(l ill l)la(dv {)a[)er; the intensity of the magnetic field was about 

 4.i>()0 ('. (;. S. units. 



If we (h) not inclose the |)hotographic plate, and if we arrang(> on it 

 several strii)s of paper or of metal to form screens, we o))s(>rve in the. 

 print of the radio-activity dispersed by the magnetic field, a species of 

 a;)sorption spectra. Va\v\\ trajectoi-y has a ditierent curvature corre- 

 sponding to rays of different speeds and having difierent penetrating- 

 powers. 



Here is an example of one of these prints, obtained in a held of 

 about 1,740 C G. S. units; the screens are a strip of l>lack ])a})er, a 

 strip of aluminum of 0.1 nun. thickness, and a stri}) of platinum of 

 0.08 mm. thickness {[\(^. S). To ol)tain a pure spectrum so that at each 

 point of the plate a bundle of raysai'e found, of which the trajectories 

 have all the same curvatui'e, the rays should Ik^ made to issue from 

 the source so as to pass throug!' -i small round <)])ening; the lesultis 

 the sam(» as the preceding one (lij,. . ). This hittei- also sliows a xcry 

 intense impression, du(^ to the secondary rays, provoked by the rays 

 which were stopped by the lead cover over tlu^ active body, and in 

 whidi was made a small opening through which the i)ui-e spectrum 

 ])assed. The al>sorption varies Avith the distance of the screen from 

 th.' ac-tive body, and the rays which are stop})ed by a screen phiced on 



"" Recemment, Mr. Rutherford a reconiui par la iiu'tlioile ('lectriqiie (|ue les rayons 

 toiu- absorbables, (jn'il a appelt's rayniis (i, rtaient l'ail)lciiu'nt dt'vies en scnt< con- 

 traire des rayons eathodiii'ies, ct assiniilablcs anx Kanalstrahlen. J'ai pii, dc luon 

 cote, nie.'Urer cette deviation par la iin'tliodc piioto<ira])lii(|ue, et inond'cr ((wc Ics 

 rayons d(; i>oioniuni sent identi(|Ucs anx rayons a du radium. (Note by Professor 

 Becquerel March n,d9(«.) 



