44 



SCIENCE. 



[N. S. Vol. XVIII. No. 445. 



that the electron theory, which has now 

 been universally accepted in the case of 

 the kathode rays, has naturally been ex- 

 tended to include the Becquerel raj^s. 



Until recently the a rays were supposed 

 to be imdeflected in passing through a 

 magnetic field. Within the last few 

 months, however, it has been shown both 

 ty Rutherford and Becquerel that these 

 rays do experience a deflection. This de- 

 flection is in the opposite direction from 

 that experienced by the /? rays, and is much 

 less in amount. An extremely strong field 

 is necessary to show the deflection at all. 

 This behavior of a rays is explained by 

 assuming the rays to consist of positively 

 charged particles moving at high speeds. 

 It appears, however, that the particles are 

 much larger, that the speed of these rays, 

 instead of being nearly that of light, is 

 only about one tenth as great. 



The third type of rays, the.j- rays, has 

 been only slightly studied; but so far as 

 investigation has proceeded the properties 

 of the ;' rays are the same as those of X- 

 rays. These rays are highly penetrating 

 and have been found to pass through sev- 

 eral centimeters of metal. 



The intensity of the rays from radium 

 is much greater than that of the rays from 

 the other active substances. Nearly pure 

 radium preparations have been made which 

 show an activity, as measured by the elec- 

 trical effect, five hundred thousand times 

 as great as the activity of metallic uran- 

 ium. Small traces of radium in minerals 

 thus add greatly to the activity of these 

 minerals, even when the amount present 

 is so small that no chemical test can detect 

 it. The electrical and photographic meth- 

 ods of testing for radioactivity are, in fact, 

 more sensitive than any chemical or spec- 

 troscopic tests yet discovered. 



The great difference between the activity 

 of radiiim and that of the other active sub- 



stances, and the fact that small traces of 

 radiimi are hard to detect, has led to the 

 thought that the activity of other sub- 

 stances might be due to the presence as 

 an impurity of some highly active element, 

 possibly radium itself. In the case of 

 uranium it seemed for a time as though 

 this view was definitely confirmed. Upon 

 precipitating a solution of uranium by am- 

 monium carbonate Crookes succeeded in 

 separating uranium into two parts, one of 

 which was redissolved by excess of the re- 

 agent, while the other remained behind as 

 a precipitate. Only a trace of the latter 

 was obtained, but it was many times more 

 active than the original uranium. In fact, 

 when tested by photographic methods this 

 uraniiun-X, as it is called, seemed to have 

 all of the original activity, while the or- 

 dinary uranium was no longer active. It 

 was found, however, that the uraniTim-X 

 soon lost its activity, falling to one half 

 its original strength in about twenty-two 

 days; while the ordinary uranium grad- 

 iially recovered its activity, regaining one 

 half of its original strength in about the 

 same length of time. 



The investigation which led to the 

 separation of uranium-X illustrates the 

 contradictions which may arise in work 

 of this kind. Crookes used the photo- 

 graphic method and obtained the results 

 just -stated. Others, repeating his work 

 by the electrical method, reached very 

 different conclusions. When tested elec- 

 trically the increased activity of Ur-X 

 was not nearly so marked, while the or- 

 dinary uranium was nearly as active as 

 ever. The separation is now seen to be 

 one which divides the active uranium into 

 two pai'ts, both of which are active; but 

 one part gives out « rays, while the other 

 gives chiefly /? rays. A similar separation 

 has been effected in the case of thorium. 

 In this case thorium nitrate is precipitated 



