CONTEMPORARY ADVANCES IN PHYSICS 69 



radioactivity was discovered. They disclose the only known instance 

 of distinct stable isotopes of an element being found separately from 

 one another in nature. Whether "ordinary lead" of atomic weight 

 207.2 is a mixture of these two isotopes, or contains still others, is as 

 yet an unsolved question. ^^ 



The three series resemble one another not only in the nature of their 

 terminal substances, but in other regards as well. The substance in 

 the radium series known as ionium, the member of the actinium series 

 called radioactinium, the member of the thorium series named radio- 

 thorium, are isotopes all three of the element 90; and these three 

 substances evolve through the same succession of transformations, 

 alpha-ray emissions and beta-ray emissions following after one another 

 in the same order. The nth. descendants of these three substances, for 

 each value of n from 1 to 6, are isotopic with one another — a statement 

 which will probably be made clearer by Fig. 1 than by these words. 

 This parallelism, which from the grandchildren of lo and RdAc and 

 RdTh onward is reflected in the names of the substances, includes also 

 the "branchings" which occur in each sequence at the substance 

 labelled C^ — radium C and actinium C and thorium C. It is limited 

 in its range, for the earlier parts of the three sequences are by no means 

 alike, while the radium sequence continues onward for three stages 

 longer than the two others. Something within the radium atom impels 

 it to continue evolving even after it has twice taken and left the atomic 

 number which it is destined eventually to take and keep, although the 

 atoms which were once actinium or thorium are contented to stop at 

 the atomic number 82 when for the second time they reach it. 



The phenomenon of branching, which I have twice casually men- 

 tioned, is worthy of a few paragraphs. It signifies that a certain pro- 

 portion of the atoms of such a substance as (for instance) thorium C 

 transmute themselves in one fashion, the remainder in another. 

 Sixty-five per cent of the atoms of ThC extant at any moment are 

 destined to emit beta-rays and become atoms of a substance ThC 

 lying one step further up the procession of the elements; the other 

 thirty-five per cent eventually emit alpha-particles and become atoms 

 of ThC" placed two steps further down the procession. Such a "dual 

 transmutation" occurs also at RaC and at AcC — an instance of the 

 parallelism just mentioned, which however does not extend to the 

 relative frequency of the two modes of transformation; 9996 out of 

 ten thousand atoms of RaC, but only three out of a thousand atoms of 



'^ Not however a definitely insoluble question, since the Thomson-Aston method 

 of resolving mixtures of isotopes (Introductioti, pp. 14-29) and measuring their indivi- 

 dual masses should be applicable to lead — that is to say, certain difficulties have thus 

 far prevented it from being applied to the very heavy elements, but these difficulties 

 may not prove insuperable. 



