278 Wells & Foote — One Hundred Years of Chemistry, 



esting observation has just recently been made by Soddy 

 that the products of radioactive disintegration appear to 

 pass in a symmetrical way through positions in the 

 periodic system, giving off a helium molecule at alternate 

 transformations until the place of lead is reached. It 

 appears, therefore, that the five vacant places in the table 

 above bismuth are probably occupied by these evanes- 

 cent elements, and it is to be noticed that all of the 

 elements that have been placed in this region of high 

 atomic weights are radioactive. 



There are some inconsistencies in the periodic system. 

 The increments in the atomic weights are irregular, and 

 there are three cases, argon and potassium, cobalt and 

 nickel, and tellurium and iodine, where a higher atomic 

 weight is placed before a lower one in order to bring 

 these elements into their undoubtedly proper places. 

 There is a peculiarity also in the heavy-metal division of 

 Group VIII, where three similar elements occur in each 

 of three places, and where the usual periodicity appears 

 to be suspended, or nearly so, in comparison with most 

 of the other elements. However, there seems to be a 

 still more remarkable case of this kind in Group III, 

 where fourteen metals of the rare-earths have been 

 placed. , They are astonishingly similar in their chemical 

 properties, hence it seems necessary to assume that 

 periodicity is suspended here throughout the wide range 

 of atomic weights from 139 to 174, where no elements 

 save these have been found. 



Several other interesting features of the table may be 

 pointed out. The chlorides and hydrides, as indicated 

 by the "typical compounds," show a regular progres- 

 sion in both directions towards Group IV. (Where the 

 type-formulas do not apply, as far as is known, to more 

 than one or two elements, they have been placed in 

 parentheses in the table given here.) It is a striking- 

 fact that the acid-forming elements occur together in a 

 definite part of the table, and that the gases and other 

 non-metallic elements, except the inactive gases of Group 

 VIII, occur in the same region. 



Atomic Numbers. — As the result of a spectroscopic 

 study of the wave-lengths or frequencies of the X-rays 

 produced when cathode rays strike upon anti-cathodes 

 composed of different elements, Moseley in 1914 discov- 

 ered that whole numbers in a simple series can be 



