ARGON. 527 



corresponding members of the other periods, and a regular grada- 

 tion of properties among themselves ; or, in other words, " the 

 properties of the elements and of their compounds are a periodic 

 function of their atomic weights." The exigencies of classifica- 

 tion, so that the elements of different periods may fall into their 

 proper places in the tabulated scheme, have left many gaps in the 

 table, which may represent elements yet awaiting discovery. In 

 fact, three such elements have been discovered since the first 

 formulation of the periodic law by Mendeleeff, and found to agree 

 very exactly with the prediction made several years previously 

 by Mendeleeff for the properties of elements which might be 

 expected to fill certain gaps. 



Now argou, if it is a monatomic element, must have an atomic 

 weight of about forty. There is, however, not only no vacant 

 place in the table for an element of this atomic weight, but the 

 properties of the elements occupying this region are wholly 

 unlike those of argon. Thus for the first time in its history the 

 periodic law would fail in its hitherto triumphant provision for 

 the results of discovery. 



A law which expresses so much undoubted truth, and which 

 has proved of such great service in the past, is not, however, to 

 be at once discredited because it seems not to provide for this 

 case. So great is the confidence felt in it, that many chemists 

 consider its apparent failure in this case a conclusive argument 

 against the monatomic character of argon. It must be remem- 

 bered, however, that the evidence for monatomicity is founded on 

 a deduction from the thoroughly established mechanical theory 

 of heat ; while the periodic law is, after all, as Prof. Riicker says, 

 " an empirical law, which rests on no dynamical foundation," for 

 which no adequate theory has yet been found. More evidence is 

 needed in the case, and more will probably soon be forthcoming. 

 Meantime the present situation will strengthen the feeling, by no 

 means new, that, while the periodic law is a grand generalization 

 containing much that is true, it is certainly not a complete or 

 final expression of the relations which exist between the proper- 

 ties of the elements and their atomic weights, but rather a first 

 approximation to the law which may ultimately be formulated. 



Whatever the outcome of these speculative issues, there can 

 be but one opinion in regard to the discovery itself. From every 

 point of view it is a masterly achievement. The elements of re- 

 cent discovery have all been metals which occur in minute 

 quantities in rare minerals. No nonmetallic element has been dis- 

 covered for nearly seventy years, and the existence of another 

 element belonging to this group did not seem probable ; still less 

 likely did it appear that such an element could be present in our 

 atmosphere. 



