102 THE EVOLUTION OF MATTER 



expels 3 a-particles. The atomic weight of uranium 

 is 238, and that found by Mme. Curie for radium is 

 226. So far so good. Radium in its further 

 changes expels 5 a-particles, and the atomic weight 

 of the end product should be therefore 206. The 

 atomic weight of thorium is 232, and, as it expels 

 6 a-particles in all, that of the end product of 

 thorium should be 208. The atomic weight of 

 ordinary lead is 207-2. The atomic weight of 

 bismuth is 208, but the writer was unable to find 

 in a special examination of over 20 kilograms of a 

 certain thorium mineral even a trace of bismuth, 

 though there was 0-3 per cent, of lead. This 

 definitely rules bismuth out. 



In the early months of 1913 a fundamental step 

 forward was taken into our knowledge of the nature 

 of matter which started from the discovery of the 

 simple complete law of elementary evolution as we 

 have come to know it in radioactive change, which 

 is largely due to two of the writer's old students, 

 A. S. Russell and A. Fleck. The expulsion of the 

 a-particle, or the /3-particle, from an atom leaves a 

 new atom with properties different from the parent, 

 but different in a very definite and striking way. If 

 the particle expelled is the a-particle, the element 

 after this expulsion invariably changes its whole 

 chemical character and passes from the place it 

 occupies in the Periodic Table to a new place, next 

 but one to it in the direction of diminishing atomic 

 weight. If the expelled particle is a /S- par tide the 

 change of place is invariably into the next place in 

 the opposite direction. After three changes in any 

 order, one a- and two /3-, a very common sequence 

 in the series, the element returns to the place it 

 first occupied. Its atomic weight is less than it was 

 by 4 units, but in its whole chemical nature and even 

 in its spectrum, it is not merely like its original 



