208 
specific gravities of analogous members in each series in- 
crease in the order of their atomic weights, it would appear 
that cerium does not belong to the same series as lanthanum 
and didymium. Moreover, considering the important posi- 
tion which £ = 69 occupies in relation to its analogues Al, Y, 
and the position which these three elements occupy in 
relation to their homologues Mg, Zn, Cd, and Na, Cu, and 
Ag, it may be doubted if £=69 should, up to the present 
time, have remained undiscovered, especially as all its ana- 
logues of the series Th, E, Y, and Al, are well known. If, 
therefore, £ = 69 be cerium, the only element missing in the 
series H3 n is x = 42, the analogue of Ga. In, and TL As 
these elements have been discovered by spectrum analysis, 
it is probable that x = 42 will also be found by the same 
means. It may however be observed, that the characteristic 
lines of the alkaline metals in the series H n, and of their 
homologues H3 n advance in the blue or violet end of the 
spectrum, towards the more refrangible parts in the inverse 
order of their atomic weights. The spectral lines of £=42 
must therefore be sought for in the violet or ultra violet 
part of the spectrum.' The high refrangibility of the lines 
which the missing element will have, may be the reason 
why it has hitherto escaped detection, as from the wide dis- 
tribution in nature of its homologues of position Ca, and K, 
in relation to their respective analogues Sr, and Rb; £ = 42 
ought to be more abundant in nature than gallium. 
From the physical and chemical relations which subsist 
among the halogens FI, Cl, Br, I, and the alkaline metals 
Li, Na, K. Rb, Cs, chemists have already justly considered 
these elements as positive and negative analogues of each 
other and of hydrogen. In accordance with this view, I 
have classified the halogens as negative forms of the series 
H?i. By assigning to these elements the positions shown in 
the table, it will be seen that besides the triad of atomic 
weights formed by Cl, Br, and I, there is a common differ- 
