226 Mr. D. C. H. Florance on 



are characteristic o£ that element and correspond to the 

 •■ characteristic radiation " of the K series established by 

 Barkla. These characteristic radiations are absorbed ac- 

 cording to an exponential law and have generally been 

 considered to be homogeneous. Recent experiments * on the 

 reflexion of X rays from the surface of crystals have made 

 it clear that these characteristic radiations may consist of 

 several types of radiation of definite wave-lengths and with 

 definite absorption coefficients in aluminium. For the 

 present we may assume that the 7 rays emitted from radium 

 C are initially homogeneous ; but the experiments described 

 in this paper show conclusively that 7 rays cease to be 

 strictly homogeneous after passing through matter. In fact 

 there is a continuous transformation taking place throughout 

 the whole range of absorbing screens. 



In the following experiments a tube filled with radium 

 emanation served as a source of 7 rays. The tube was first 

 surrounded with lead O084 cm. thick, so that the two soft 

 types of 7 rays from radium B were completely absorbed, 

 and the emergent 7 rays were due chiefly to the 7 rays of 

 radium C and to a certain extent to the 7 rays of radium B, 

 whose absorption coefficient in aluminium was 0*51 cm. -1 . 

 Then a screen of lead 1*14 cm. thick was added, which is 

 sufficient to absorb almost completely the 7 rays from 

 radium B. The penetrating radiation which emerges con- 

 sists of unchanged 7 rays from radium C, together with a 

 radiation that has been in some manner transformed in 

 passing through the absorbing screen. However much 

 additional material the rays pass through there is always 

 present with the primary radiation a transformed radiation 

 indicating that there is a continuous change in type of the 

 primary beam in each successive absorbing screen. The 

 emergent radiation is always complex ; but since the loss of 

 energy in any layer is always proportional to the energy 

 entering that layer, the 7 rays of radium C have been 

 described as being " homogeneous/" But it must be under- 

 stood that the term " homogeneous " in this case has a 

 different meaning from that applied to the 7 radiation from 

 radium C which has not passed through absorbing screens. 

 It is probable in the latter case that the radiation initially 

 does not contain that distribution of softer types of radiation 

 which are always present after the 7 rays have passed 

 through matter. It was shown by Soddy (loe. cit.) that the 

 7 rays of radium were exponentially absorbed by lead, 



* W. II. & W. L. Bragg, Prop. Hoy. Soc. July 191:3 Moselev & 

 Darwin. Phil. Mag. July L9J8. 



