CH. xviii.] RADIO-ACTIVITY 173 



Eutherford has examined the deflexion of alpha 

 particles from radium in a careful manner. Previous 

 experiments, such as his own and those of Becquerel 

 and Des Coudres, were made on a thick layer of 

 radium ; but under these circumstances the particles 

 are projected with a considerable range of velocity. 

 To obtain homogeneous radiation it is necessary to 

 use a very thin layer ; and Rutherford employed 

 for this purpose the product Radium C, namely, 

 part of the active deposit which appears on a fine 

 wire exposed for some hours to radium emanation. 

 This is a deposit of utterly imperceptible thickness, 

 undetectable by any means save radio-activity ; and 

 it consists of Radium A, B, and C. The activity of 

 Radium A disappears in about a quarter of an hour ; 

 Radium B emits no rays ; so Radium C alone is left 

 active, and it emits only alpha rays. It is true it 

 dies away in about a couple of hours, but there is 

 time enough for an experiment. The method em- 

 ployed is like this : 



Rays from the wire pass through a narrow slit, and 

 then on to a photographic plate in a vacuum ; a 

 uniform magnetic field is applied in a direction 

 parallel to the slit, so as to curve the rays ; this field 

 is reversed every ten minutes, so that on developing 

 the plate two narrow parallel lines are observed, the 

 distance between which represents twice the de- 

 flexion ; their sharpness shows that the rays were 

 homogeneous. The path corresponding to a magnetic 

 field of 9470 C.G.S. units had a radius of curvature 

 equal to 42 centimetres. 



Electric deflexion was not then applied ; but, 

 estimating the number of particles expelled from the 

 Radium C as 6*2 x 10 10 per second, and the heating 

 effect as 31 calories per hour, Rutherford calculated 





