878 Dr. J. W. Xicholson on a Structural 



just before copper in the Periodic Table, and the hypo- 

 thetical element has been predicted to have an atomic weight 

 about equal to 63. The formula 



X 1 =2{He 2 (PfH) 3 .Nu 2 (P£H) 3 } 



which we select, in accordance with necessary selections for 

 other elements, gives an atomic weight 62*88. The unknown 

 gas is called X x , and it is possible that the incomplete group- 

 ings involving He 2 , Nu 2 are a criterion of instability. The 

 possibility of a decomposition of this gas into the other inert 

 gases with necessary stability is evident from the formula. 



Returning to the argon group, we note that the gas pre- 

 ceding niton has a predicted atomic weight of about 176, by 

 the usual method of the Periodic Table. This value has been 

 mentioned, for example, by Sir William Ramsay. Calling 

 it X 2 , we may try a formula 



X 8 = 5{He 2 (PfH) 3 .Nu 4 (PfH) 3 }, 



giving the atomic weight 173*5. It contains a possibly 

 unstable group, and may readily decompose into the inert 

 gases of the atmosphere. 

 An alternative formula is 



X 2 = 2{2Nu 4 (Pf H) 3 . 3He 2 (PfH) 3 } 



with an atomic weight 175. The latter is preferable, but a 

 real decision could only come with a knowledge of the gas. 

 The second formula may illustrate a relation with niton, and 

 it notably combines the double grouping or the neon series 

 and the quintuple one of the argon series, besides giving a 

 better value for the atomic weight. 



It is worth the trouble to continue these speculations 

 regarding unknown gases, though briefly, since one of them 

 is generally believed to be the emanation from actinium. 



The next vacancy in the neon group immediately precedes 

 silver, and the corresponding gas should have an atomic 

 weight of about 106. We suggest 



X 3 = 2{He 4 (PfH) 3 . 2Nn,(Pf H),} 

 or 



X 3 = 2{Nu 4 (P£ H), . 2He,(PfH),} 



with an atomic weight 105*6. These might be different 

 elements, with the same groupings arranged differently. 

 Examples of this are not unknown, and the case of uranium 

 and uranium X may be cited. 



