CONTEMPORARY ADVANCES IN PHYSICS 67 



all the isotopes of each element are stable, the elements have individual 

 names and the isotopes are designated only by their masses. The 

 elements 81, 82 and 83 have some isotopes which are radioactive and 

 others which are not; thus the word "radioelement" is misleading, 

 and should be replaced by "radioactive isotope." Consistency 

 indeed requires that one speak of the successive members of a family 

 of radioactive substances not as consecutive elements, but as con- 

 secutive isotopes of diverse elements. At this point, however con- 

 sistency almost ceases to be a jewel. I can find no satisfactory com- 

 promise, and will hereafter refer to the various radioactive materials 

 simply as "substances" — so bringing to an end this long analysis of 

 words, which is justified only in so far as it may have concentrated the 

 reader's attention upon the facts underlying them. 



We return to Fig. 1. 



The radioactive substances are grouped into three main lines of 

 descent or sequences, commonly called series. Each of these throws 

 off one or two branches, which however cannot be followed far; these 

 I will discuss further on, pausing here only to mention that one of the 

 three main sequences, the actinium series, is believed by many to branch 

 in this manner out of the uranium-radium series. This however is not 

 certainly established, and it is suitable to regard these two and the 

 thorium series as independent sequences, which between them comprise 

 all the known radioactive isotopes among the elements.^^ 



Uranium and thorium, the first elements of the series to which they 

 have given their names, are even yet after all the aeons of the earth's 

 existence to be found in abundance among its rocks. This practically 

 proves that uranium, at least, disintegrates with exceeding slowness; 

 for all the other known elements are lighter than it is, and consequently 

 there is none of them out of which the steadily-dwindling supply of 

 uranium might be replenished by transmutation. We shall presently 

 learn methods of estimating the duration of uranium, by which it is 

 shown to be truly colossal. 



The atomic weights of uranium and thorium are known, and amount 

 to 238.18 and 232.12 respectively. From these it should be possible 



ticular isotopes of the elements 84, 88, 89, 90 and 91 respectively, but are sometimes 

 used as names for these elements — another dangerous source of misunderstanding. 

 The name niton was formerly used for the isotope radon of element 86, and might well 

 be used for this element now that the isotopes are individually named. 



^^ Apart from the elements potassium and rubidium, which will continually demand 

 to be mentioned as exceptions unless they are disposed of once for all at this point. 

 Let it be stated, then, that these elements emit electrons, so feebly however that they 

 are much less active than even uranium, which ranks among the least radioactive of 

 all the known radioactive substances; and that no one has identified the substances 

 into which they are transmuted, though presumably those are isotopes of calcium 

 and strontium respectively. Cf. an account of the radioactivity of these elements 

 by A. Holmes and R. W. Lawson: Phil. Mag. (7) 2, pp. 1218-1233 (1926). 



