126 CONCEPTION OF THE CHEMICAL ELEMENT 



substance to the title of element, as in settling dis- 

 putes as to what multiple of the equivalent was to 

 be adopted as the atomic weight, the periodic law 

 became the court of appeal. Did a claimant to the 

 title of element fit into a vacant place in the family 

 of related elements? If it did, not only was there 

 no doubt as to its atomic weight, but it certainly 

 could scarcely be an ordinary compound or mixture. 

 Whatever the elements were, it was clear that they 

 were all of a class, the limits of chemical analysis, 

 and, if complex, then all probably of the same kind 

 of complexity. 



Incidentally, also, the periodic law showed that 

 although there was a connection between atomic 

 weight and chemical character, there were exceptions, 

 like tellurium and iodine, where the atomic weights 

 appeared to have been reversed. This made it 

 perfectly plain that it was merely a chance that no 

 two elements happened to possess the same atomic 

 weight. Dalton, as we shall come to describe, 

 discovered in the atomic weight not merely a new 

 atomic property, but a new class of atomic property 

 which, until the present century, remained the only 

 one of the kind known, and is concerned with a 

 different region of the atom from that to which 

 physical and chemical character, position in the 

 periodic table, spectrum, and other identifying 

 characteristics are to be referred. 



The discovery of spectrum analysis led to the 

 recognition of many new elements, caesium and 

 rubidium, thallium, indium, helium, and gallium all 

 being so recognised before anything at all was known 

 as to their other properties. In each case unique 

 spectrum was later found to correspond with unique 

 chemical character except for the argon gases, all 

 characterised by absence of chemical character and 

 unique atomic weight. 



