592 



SCIENCE 



[N. S. Vol. XLI. No. 106O 



ever two or more radio-elements fall in the same 

 place in the Periodic Table, then, independently 

 of all considerations as to the atomic mass the na- 

 ture of the parent element, and the sequence of 

 changes in which they result, the elements in ques- 

 tion are chemically non-separable and identical. 

 As will later appear, this identity extends also to 

 most of the physical properties such as volatility 

 and spectrum reactions.^ 



To express this "newly revealed com- 

 plexity of matter," Soddy has suggested 

 the word isotope. A group of two or more 

 elements occupying the same place in the 

 periodic table, differing in atomic weight 

 yet chemically non-separable, is isotopic. 

 There are possibly seven such elements 

 isotopic with lead. Eadium is one of four 

 isotopes. The chemistry of thirty-seven 

 radio-elements is thus reduced to a smaller 

 number of about ten types. 



Two fundamental changes in the old 

 views as to the system are indicated here. 

 First, the position of an element is not 

 fixed but can be changed in either of two 

 ways — by a change in valence (which, as is 

 well known, can be brought about in vari- 

 ous ways) , and again by disintegration due 

 to ray-emission. Secondly, more than one 

 element can occupy a given position in the 

 system. This is independent of the atomic 

 weight, but such elements are chemically 

 inseparable. This involves the giving up 

 of all idea of the properties being functions 

 of the atomic weights and necessitates the 

 formulation of the law anew. 



The place occupied by an atom is not 

 solely determined by its mass but by its elec- 

 trical content as well. According to Soddy, 

 the system represents the chemical char- 

 acter of matter as the function of two 

 variables instead of one. The electrical eon- 

 tent is the essential variable in horizontal 

 columns and mass is the essential in ver- 

 tical columns. 



It is somewhat early to raise the question 



2 Soddy, ' ' The Eadio-elements and the Periodic 

 Law," p. 6. 



as to whether all elemental atoms are the 

 result of disintegration processes, or, con- 

 versely, of synthesis, but in any case the 

 old puzzle remains as to their great irreg- 

 ularity in weight relations if the most 

 accurate chemical determinations are to be 

 relied upon. If the time should arrive 

 when they could be calculated, chemists 

 would naturally return to hydrogen as the 

 standard. Certainly, at present these 

 weights present no simple synthetic rela- 

 tions. 



An explanation of this is perhaps at 

 hand if the view of Soddy (and of Crookes 

 at an earlier period and from a different 

 standpoint) is accepted, namely, that in 

 atomic weight determinations it is not a 

 natural constant that is obtained but a mean 

 value of non-homogeneous masses. In other 

 words, the weight may represent the aver- 

 age of various isotopic atoms and not the 

 absolute weight of identical atoms. 



It is very fortunate that the simple expe- 

 dient of arranging the elements in the order 

 of their atomic weights could give the early 

 workers so nearly correct a view of the 

 periodic system. It would probably have 

 remained hidden for a long time if this 

 had not been so prominent a factor in deter- 

 mining the proper sequence. There is un- 

 doubtedly a proper sequence. This has 

 been settled hitherto chiefly by considera- 

 tion of the atomic weight, but also by exam- 

 ination into the relationship existing be- 

 tween the elements. For instance, the 

 order of atomic weights would be iodine 

 and then tellurium, but chemically tellu- 

 rium belongs to Group VI. and iodine to 

 Group VII. Therefore, the atomic weight 

 order is reversed. 



The sequence numbers of the elements, 

 or atomic numbers as they are called, as- 

 sume a new practical and theoretical im- 

 portance. "Within twenty years after the 

 announcement of the periodic system, some. 



