794 Proceedings of the Royal Irish Academy. 



respectively. Assuming that a similar simple ratio holds good in the 

 progressions of atomic weight that constitute the cycles higher than 

 the fourth, the progression from caesium to the element at the head of 

 the sixth cycle is probably six times sixteen, or about 96, which makes 

 the atomic weight of this element about 226. We may, therefore, 

 anticipate the future discovery of an alkali metal having approximately 

 this atomic weight. Such an element would have a density of 2"5 or 

 thereabouts, and a corresponding atomic volume of about 90. Its 

 melting point would be low, and its chemical affinities intense, and, 

 as is the case with caesium, the metal probably would not be reduced 

 by ignition of the carbonate to whiteness with charcoal. Grranting the 

 existence of this element and of a halogen analogous to iodine, the fifth 

 cycle is terminated, and thorinum and uranium form part of the sixth. 

 This implies the existence of an alkaline earth metal with atomic 

 weight about 230, and of an earth metal analogous to Sc and Tt to 

 precede thorinum. Thus : 



Eb, 



Cs, 



Alkali metal. 



Sr, 



Ba, 



Alkaline earth metal. 



Tt, 



— 



Earth metal. 



Zr, 



— 



Th. 



Thorinum and uranium thus come within the earth region of the sixth 

 cycle, and the coloured solutions of uranium are normal phenomena 

 falling under the law of atomic volume. 



The platinoid metals of the sixth cycle probably closely resemble 

 their atom analogues of the fifth, and also have close lateral affinities 

 with each other. Judging by analogy, we may expect to find these 

 higher platinoid metals associated in small quantities with their lower 

 atom analogues, and to experience considerable difficulty in separating 

 them from the latter. Their unsuspected presence in iridium and 

 osmium may possibly account for the upward displacement of these 

 metals in the fifth cycle. In the same way the presence of a higher 

 atom analogue of tellurium (atomic weight about 214) may account for 

 the distortion of the fourth cycle by this element. 



Brauner has recently examined the oxides of the metals lanthanum, 

 cerium, and didymium, and has assigned to these elements the positions 

 in the fifth cycle respectively analogous to the elements yttrium, zir- 

 conum, and niobium in the fourth. That this is the true sequence and 

 atom analogy of the cerite metals now admits of little doubt, but, at 

 the same time, the progression from yttrium to niobium constitutes a 

 far larger portion of the fourth cycle than the progression from cerium 

 to didymium does of the fifth. The cerite metals all occupy the earth 

 position in the fiith cycle, and in their general properties are elements 

 of the pure earth type, and, as such, strictly analogous to aluminum ; 

 and facts thus justify L. Meyer's conception that all three are ana- 

 logues of the earth elements. They may be said to be cycle analogues 

 of aluminium ; and the series analogues of yttrium, zirconium, and 



