4 ROYAL SOCIETY OF CANADA 



the gas. The ionization produced per cm. of path of the gas by each a 

 particle was approximately the same over the whole range, and was 

 found to end fairly abruptly. Each of these sets of rays of definite 

 range in air was found to correspond to the rays from one of the four oc 

 ray products present in radium in radioactive equilibrium. For 

 example, the rays from radium C passed through about 6.7 cms. of air, 

 when the ionization ceased. The corresponding distance of the rays 

 from radium itself was about 3 cms. These results indicated that each 

 product of radium emitted a particles of the same velocity, but that 

 this velocity varied considerably for the rays from the different products. 

 As Bragg has pointed out, these results confirm in a novel and interest- 

 ing way the theory of succe.-sive changes -which has been advanced from 

 data of quite another character. 



By passing the rays through an absorbing screen of thickness à, 

 the range in air was found to be reduced by the distance pd when p 

 was the density of the screen compared with air. This is an expression 

 of the fact that the absorption of the rays is proportional to the density 

 of matter traversed. In a thick layer of radioactive material of one 

 kind, the rays emitted into the gas come from different depths, and 

 consequently pass through a distance in air varying from zero to the 

 maximum range corresponding to the rays from a thin film of that 

 material. It is thus obvious that the a particles escaping into the 

 gas will have different velocities, that is, the rays will be complex. In 

 radium in radioactive equilibrium, which contains four distinct pro- 

 ducts which emit n particles, the rays are still more complex in 

 character as each set of rays has all ranges in air between zero and its 

 maximum. 



In order to obtain results of a de'inite character, it is thus advisable 

 not to use radium itself as a source of rays, but a thin film of radio- 

 active matter of one kind, so that the rays all escape into the gas with 

 the same velocity. 



This condition is fulfilled by using, as a source of rays, a wire made 

 active by exposure to the radium emanation. The active deposit on 

 the wire after removal contains the three products radium A, B, and C. 

 Since radium A is half transformed in three minutes, the activity due 

 to it has practically disappeared in the course of fifteen minutes. The 

 ravs, then, are only emitted from the one product radium C, since 

 radium B does not emit rays at all. The active deposit on the wire 

 is so thin that no absorption takes place in tlie active matter itself, so 

 that all the rays escape without change in thoir velocity. The «' 

 particles projected into tho wire arc absorbed completely by the wire 

 itself. 



