222 
CHEMISTRY: W. D. HARKINS 
If attention is now turned to the heavier elements as shown in the 
model, it is seen that the five unknown elements eka-caesium, eka- 
manganese 1, eka-manganese 2 (dwi-manganese), eka-iodine, and eka- 
neodymium, have odd atomic nimibers. (There is some doubt as to the 
discovery of thulium 2.) Not only are the unknown elements odd nxma- 
bered, but among the radio-active elements, if the most stable isotope 
of each element is used for the comparison, the odd numbered elements 
are much less stable than the adjacent elements of even number. 
If we consider the rare-earths — the elements which are most similar 
chemically, while at the same time their atomic numbers change in 
steps of one — the same result is obtained. In the following table, which 
includes, besides the rare-earths a number of elements adjacent to them, 
the letter c indicates common in comparison with the other elements in 
the table, and r indicates rare, cc represents very common, etc. The 
comparison is only a rough one, but it is suificiently accurate for the 
purpose for it indicates that in every case the even numbered element 
is more abundant than the adjacent odd nimibered element. 
TABLE 2 
ATOMIC 
mnsiBER 
ABUNTANCE 
ELEMENT 
ATOMIC 
NUMBER 
ABUNDANCE 
ELEMENT 
55 
c 
Caesium 
63 
rr 
Europium 
56 
ccc 
Barium 
64 
r 
Gadolimium 
57 
c 
Lanthanum 
65 
rrr 
Terbium 
58 
cc 
Cerium 
66 
r 
Dysprosium 
59 
r 
Praseodymium 
67 
rrr 
Holmium 
60 
c 
Neodymium 
68 
r 
Erbium 
61 
rrr 
Unknown 
69 
rr 
Thulium 
62 
c 
Samarium 
The above results may be summarized by the statement that in the 
evolution of the elements much more material has gone into the even 
nimibered elements than into those which are odd, either because the 
odd Qumbered elements are less stable, or because some constituent 
essential to their formation was not sufficiently abimdant, or both. 
It is easy to see too that most of the material has been used up in 
the formation of the lighter elements. Table 2 shows that in the meteor- 
ites the most abundant elements are oxygen in series 2, the elements of 
series 3 except neon, and the members of the first eighth group triad 
(iron, cobalt, nickel). Clarke^ has found that just these same elements 
are the most abundant in the Kthosphere, although in the Kthosphere 
potassium and calciiun in series 4 are also moderately abundant. If the 
lithosphere were considered alone it might be considered that the abun- 
