211 
The classification of uranium presents some difficulty on 
account of the fewness of its analogies with other elements, 
but there can he little doubt that the atomic weight as- 
signed to U = 120, until recently, is much too small ; as there 
are no elements with atomic weights so low, correlated with 
specific gravities so high as that of U = 18*3. From a study 
of the chemical combinations of this element, Mendeleef has 
assigned to it the atomic weight of 240,*' or double the num- 
ber formerly received, and which number I have adopted. 
The admission of this high atomic weight, however, separates 
uranium from chromium, molybdenum and tungsten with 
which it has been classified, as there are no elements of ap- 
proximately the same high specific gravities as tungsten = 
18*26, and uranium = 18*3, correlated with so great a differ- 
ence of atomic weights as U = 240, and W = 184. From the 
fact that the highest places in all the series, except that in 
H4 n, are filled up with their highest members, and that 
uranium is generally found in combination with the mineral 
species yttrotantalite, fergusonite , polykrase, pyrochlore, 
pyrrhite, containing elements of the series H3 n on the one 
side, and in combination with minerals containing elements 
of the series IFm on the other, I have classified uranium 
as the highest form of H4^. The two lower forms of H4 n, 
as will be seen from the table, are missing; but, assuming 
that titanium is the highest member in a triad with the 
missing elements, the atomic weights of the latter are 16 
and 32, isomeric with oxygen and sulphur. It may, how- 
ever, be surmised that no elements now exist to fill the gaps 
in the series, as they may have become extinct by absorption 
into titanium and its analogues, or by transformation into 
the negative forms of H2 n. 
The elements which I have classified as forms of H 6n are 
only three in number, and the atomic weight of chromium 
= 52*2 establishes its position as the third member of the 
* Ann. Chem. Pharai. Suppl., viii., 178 — 184. 
