1922] Isotopes 117 



taken the atomic weights fell very close together and it was within 

 reason to think that the probable error lay there. The two chief in- 

 stances were those of Te and I; Co and Ni. During the following 

 years many determinations were made to settle this discrepancy. The 

 differences between them were under one-half of a unit but they were 

 real and persistent. Some suggested the existence of companion ele- 

 ments which could not be separated. Indeed Kriiss announced the 

 discovery and separation of such a companion element with nickel 

 which, if removed, would leave the atomic weight of nickel greater 

 than that of cobalt and thus justify Mendeleeff in so placing it in the 

 series. Kriiss died shortly afterwards and his supposed discovery has 

 never been confirmed. The proof that Mendeleeff was right in placing 

 these elements as he did was finally brought about through the reve- 

 lations of radioactivity. This much was evident from the first, how- 

 ever, namely, that each element had its definite position in a series, 

 or place in a system not artificially arranged but forming a natural 

 order. In this orderly arrangement two factors decide the position 

 of the element. These are the mass and the electrical content. That 

 the elements were electrically charged was recognized by Davy within 

 a few years after the announcement of the atomic theory. 



The clear exposition of the inter-relationship of the elements and 

 their arrangement in a definite order practically placed their com- 

 posite nature beyond question and their genesis became a subject of 

 speculation. Of course Prout had been the first to enter this field, 

 but so far as he was concerned it was a sheer guess without foundation 

 in fact or logic and should never have received the attention given it. 

 So, too, the later hypotheses lacked fundamental data, though some 

 of the guesses were shrewdly near the developments of these latter 

 years. 



Wliile it was definitely settled that each known element had its 

 definite place in the series, no especial import was placed upon the 

 relative number of this position. This is now known as the atomic 

 number and can be determined accurately by physical means. It was 

 first accomplished by radioactive methods but Moseley's marvelous 

 application of the defining power of quartz crystal to the definition 

 of the x-rays emitted by each element under a bombardment by nega- 

 tive elections has superseded all other methods. The spectra so ob- 

 tained reveal an orderly shifting of certain groups of lines. Begin- 

 ning with any one element, the position of all others in the series can 



