REVIEWS 779 



and to Soddy belongs the credit of the first complete statement of the unifying 

 principle. A little over a year ago, chemists regarded the elements which 

 had been discovered through their radio-activity as being mostly extraneous 

 to the periodic law — chemical sports, whose behaviour seemed little likely to 

 prove amenable to classification. But with the publication, in February of last 

 year, by Fajans and by Soddy, of the principle set out in this book, the chemical 

 relations of the radio-elements with each other and with ordinary elements were 

 suddenly revealed. 



The loss of an a-particle by an atom leaves a residual atom which weighs 

 four units less, and belongs to a group two places back from the parent in the 

 periodic classification ; whilst the loss of a /3-particle leaves a residue of the 

 same atomic weight belonging to the next higher group. The result of this is 

 that frequently more radio-elements than one must be allotted the same space in 

 the table, and sometimes a radio-element falls into a space already occupied by a 

 common element. There thus arise clusters of elements of slightly different 

 atomic weights, in each of such spaces ; and the author gives the members of 

 such a cluster the convenient name "isotopes." Moreover, it is found that the 

 members of an isotopic cluster cannot be separated chemically from each other, 

 at all events by the means which have been resorted to. 



It therefore becomes necessary to modify the notion that in the Periodic Table 

 the rule is " one space, one element," and to recognise that what are ordinarily 

 taken to be homogeneous elements may in some cases be mixtures of stable 

 isotopes. Furthermore, the range of atomic weights within a given isotopic 

 cluster may be great enough to overlap that of its next-door neighbour ; and so, 

 through radio-active instability of some members of each cluster, it could happen 

 that the order of atomic weights of the two spaces would become inverted. 

 Irregularities of atomic weights, such as that between tellurium and iodine, thus 

 receive a tentative explanation. 



On the radio-active side, it may readily be believed that the classification is of 

 great aid in elucidating the mechanism of transformations. 



It may be thought by many chemists who have followed the experimental 

 evidences for the theory that its upholders take rather too rigid a view of the 

 similarity of isotopes. That the members of a cluster are strongly alike in the 

 chemical tests to which they have been subjected, nobody would deny ; but it is 

 a bold step from this to a statement that they are chemically identical. Further, 

 it may be urged that such a statement restricts the admittedly great extension of 

 our views which the general theory gives, by excluding from the category of 

 isotopes the one case which almost any chemist would now be willing to include — 

 the rare earths. Mr. Soddy mentions this case, but it must be said that his 

 discussion of it is not quite satisfying. Modern methods of following rare-earth 

 separations are extraordinarily delicate, yet the difficulties which are entailed in 

 separation necessitate far more fractionations than have ever been used for radio- 

 element separations ; and one might hazard the remark that if, let us say, any pair 

 of the most closely related rare earths had been tested only by as few fractiona- 

 tions as have been carried out with radio-elements, they might easily have been 

 called " inseparable." The rare earths are surely isotopes, that is, they occupy 

 only one space in the table— indeed, Mr. Soddy seems practically to indicate it — 

 and they are extremely alike in behaviour. Whether they owe their origin to 

 some bygone radio-active series is an interesting matter for speculation. One 

 would have less doubt of the absoluteness of the inseparability of isotopes were it 

 not for the fact that adsorption-effects seem to be too lightly put aside, and both 



