204 
It will be observed that there are three elements missing 
in this group, the atomic weights of which can be predicted 
in like manner with those of the missing elements in the 
preceding groups. The table also affords the means of cor- 
recting and determining the atomicities of elements of the 
series which, from their rarity, have not been sufficiently 
investigated. It will be further observed that, besides the 
similar numerical relations of the members of this group 
with those shown in tables II. III., the atomic weights are all 
multiples of 3, and are classified accordingly as forms of H3^. 
The spectral reactions of this series of elements are re- 
markable from the oxides of carbon, and erbium, giving a 
spectrum of lines at low temperatures, and b}^ the simplicity 
of the spectral lines of indium and thallium in the more 
refrangible parts of the spectrum. The atomic weights of 
C, Al, Tl, and Th, are indentical with those generally re- 
ceived, and afford presumptive evidence that the atomic 
weights of the intermediate members are equally correct. 
It will, however, be observed that the atomic weights of 
yttrium and indium are double the accepted numbers ( Y = 
6T7, In = 75'G); but in regard to the latter element, it has 
not yet been definitely agreed which multiple of 37'6, the 
original determination, shall be the classical one, as the 
atomicity has been fixed by different chemists at 75‘6, 113, 
and 150, the number assigned to it in the table. The rela- 
tions which the double atomic weights of In, and Yt, have 
to each other, and with their homologues of position Cs, Ba, 
and Ag, Cd, in tables II. III., render it highly probable that 
the atomic weights of Y and In, in the table are correct. 
For similar reasons it is probable that the atomic weight ot 
erbium will be found to be 177. It is only very recently 
that any investigations of the atomic weight of this rare 
element have been made, from the difficulty attending its 
isolation from yttrium, with which it is found associated in 
nature. According to some chemists, the atomic weight of 
