ElvISHA MirCHli^hh SCIENXIFIC SOCIETY. 3'6 



106 elements if a sub-period between hydroo^en and 

 lithium be granted. Meyer's table contains 79 spaces, 

 or counting- a similar hydrog^en period 86 in all. If 

 helium and its mysterious companion really have the two 

 atomic weights assig-ned them, the assumption of this 

 hydrogen g^roup will become necessary. If MendeleeflF is 

 rig-ht, then not more than three-fourths of the elements 

 are at present known to us. The vacant space allowed 

 by Meyer for new discoveries is much more reasonable 

 and probably not much in excess of the demand of the 

 near future. 



.Of the 74 recorded by Clarke less than one-half have 

 the atomic weig^ht determined with accuracy to the first 

 decimal place. As Clarke sa^^s: "In most cases even the 

 first decimal is uncertain, and in some instances whole 

 units may be in doubt". 



As the entire arrang-ement is dependent upon the atomic 

 vveig-hts it is manifest that the doubt attaching- to those 

 in use seriously retards the development of the system. 

 Some points of great interest must be left entirely in 

 abeyance until data for accurate reasoning- are at hand. 

 The atomic weig'hts of the first third of the elements are 

 among- the best known and this justifies the arrang-ement 

 of those imperfectly determined. We would otherwise 

 scarcely be justified in laying- much stress upon the pe- 

 riodic arrang-ement. 



The imperfectly known atomic weig-hts makes it im- 

 possible to assig-n positions in the system to some of the 

 rare earth elements and certain of those more recently 

 discovered. If some of the atomic weig-hts assig-ned those 

 elements at present are even approximately correct then 

 it seems to be impossible to g-ive them their proper places 

 in any of the more prominent tabular arrang-ements of 

 the S3^stem. 



This matter of the tabular arrang-ement of the elements 

 is one which is very far from settled at present. 'Men- 

 deleefl oflfers two arrang-ements, one in vertical, the other 



