CONTEMPORARY ADVANCES IN PHYSICS 61 



This is the first half of the displacement-law of Fajans and Soddy. 

 It signifies that the emission of a-particles by a radioelement is the 

 sign of a transmutation of that element into the next hut one of those 

 preceding it in the procession of the elements. If the properties of this 

 latter element are known already, the law can be tested with all the 

 accuracy desired. Polonium stands two places after lead in the pro- 

 cession; it emits alpha-particles; its atoms should turn into atoms 

 possessing all the chemical qualities of lead, and they do. If a radio- 

 element which lies two steps ahead of an element not previously known 

 is discovered to emit alpha-particles, we are still not without informa- 

 tion as to the qualities of the element into which it should transmute 

 itself. For if we know the column of the Periodic Table in which the 

 original element lies, we know also the column in which the element 

 two steps ahead of it should lie; and the chemists know what features 

 are common to all the known elements of that column and presump- 

 tively extend also to the unknown one. Radium lies in the second 

 column of the Periodic Table; it emits alpha-particles; it should be 

 transmuted into an element lying in the "zero" column. That 

 element was not known until after radium was discovered; but it was 

 known that the other elements of the zero column are inert gases, and 

 consequently that the one into which radium transmutes itself 

 should be an inert gas. This is verified; and as a general rule it is 

 verified that when a radioelement emits alpha-particles the substance 

 left behind possesses the particular chemical features of the elements 

 belonging to that column of the Periodic Table to which the element 

 two steps preceding the original one belongs. From this fact of ex- 

 perience it is only a short step to the first part of the Fajans-Soddy 

 displacement-law — and a step which is put quite beyond criticism by 

 the relations presently to be cited which connect the atomic weights 

 of the radioelements. 



The second half of the law relates to the other radioelements, those 

 which eject electrons from their nuclei. When an atom of atomic 

 number Z emits an electron from its nucleus, the nuclear charge in- 

 creases to (Z + \)e, which is sufficient to hold another electron beyond 

 the Z electrons of the original family. The atom does pick up another 

 electron which enters into the circumnuclear set {not into the nucleus) ; 

 and it becomes an atom of nuclear charge (Z + \)e and (Z + 1) 

 orbital electrons. The radioelement changes over into another which is 

 one step farther up the procession of the elements. This is the second 

 half of the displacement-law of Fajans and Soddy. 



The evidence for this second part is extensive; but on the whole 

 it is not so imposing as the evidence for the first part. Largely this is 



