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XX. Notices respecting New Boohs. 



The Chemistry of the Radioelements. Part II. The Radio elements 

 and the Periodic Law. By Frederick Soddy, F.R.S. Pp. 46. 

 London : Longmans, Green & Co. 1914. 



TT is not too much to say that the generalization described in 

 -*- this volume (which forms one of the Monographs on Inorganic 

 and Physical Chemistry edited by Dr. Alexander Findlay) is only 

 second in importance to Mendelejeff's great generalization, of which 

 it is an extension and completion. The increasingly complicated 

 series of transformations which radioactive bodies have been found 

 to undergo are now reduced to a great simplicity. The credit for 

 the discovery which has reduced the chaos to order is shared by 

 A. Fleck, A. S. Russell, and K. Fajans. Each expulsion of an 

 alpha particle causes the element to shift two places of the periodic 

 table so as to diminish the mass by four ; while the expulsion of a 

 beta particle shifts the element one place in the opposite direction 

 and without any sensible change in mass. The result of the 

 transformations is that more than one stage can occupy the same 

 position in the periodic table with very little difference in the 

 atomic mass. Thus the end product of the transformations of 

 radium has long been suspected to be lead. But radium B has 

 also affinities to lead, and so has radium D. The old idea that for 

 two elements to occupy the same column in the periodic table 

 their atomic masses must differ by at least sixteen units has been 

 sacrificed ; in the new classification as many as six are placed in 

 the lead group with atomic weights ranging from 206 to 214, and 

 probably two more (AcB and the end product of actinium) should 

 be added thereto. No one looking at the table of changes on p. 3 

 can dispute that the gain in classification is so lar^e as to justify 

 the sacrifice that has been made of old ideas. But the matter 

 does not end there. These elements that occupy the same column 

 (and practically the same place in it) as lead have also similar 

 chemical properties to lead. Dr. Soddy gives the name isotope to 

 such a group of elements. Different isotopes are not separable 

 by chemical means. Dr. Soddy regards this as expressing an 

 essential identity ; that is as expressing something more than the 

 present limitations of the art of chemical analysis. On this 

 point opinions will no doubt differ. The majority of investigators 

 will probably be content with recognizing the extreme difficulty 

 of the separation of isotopes from one another ; and will eschew 

 the use of the word identity where any differences whatever are 

 discoverable. Be this as it may, the fact that a number f of 

 elements are now known which are, for chemical purposes, 

 indistinguishable from lead, is a fact of supreme importance. It 

 revolutionizes old ideas as to the meaning of experimentally deter- 

 mined atomic masses ; for commercial lead even after purification 

 is probably a mixture of isotopes, and the value of its atomic 

 mass will simply be a mean of those of the isotopes present. In 



