203 
into all the members of the series H2 n, and may be regarded 
as the product of the first, second, or third powers of H2. 
While the property of quantivalence would appear to be 
correlated with the number of hydrogen particles in the 
typical molecules from which the elements were evolved 
and is a valuable aid in the classification of elementary- 
species, this property in the present state of knowledge is 
not in many cases sufficient, of itself, to indicate the group 
to which an element belongs. This will be seen from the 
O 
recognised bivalency of copper and mercury, and by the 
doubtful quantivalence of silver, and by analogy of sodium, 
all of which belong to the series H n. That tetratomic 
lead = 208, is a member of the group H2 n, is shown by the 
isomorphism of its oxide, carbonate, and sulphate, with the 
oxides, carbonates, and sulphates of barium, strontium, and 
calcium, besides which there is no other place vacant in the 
system of elements where one with the atomic weight and 
physical properties of lead would fit. 
Were it not for the analogous physical properties, and the 
numerical relations subsisting among the elements grouped 
as forms of H3 n, their classification from the property of 
quantivalence alone would have hardly been possible. There 
can, however, be little doubt that aluminum, yttrium, erbium, 
and thorium are rightly classified together, and that indium 
and thallium are true analogues of each other. As consider- 
able interest attaches to this group at the present time, on 
account of the recent additions which have been made to it 
by the aid of spectral analysis, I here show the atomicities 
of its members in a separate table, calculated on the same 
principle as those in tables II. III. 
IY. 
0.0. 12= C = 12 
1 x 27 . 0 =A1= 27 
2x27-12 = — = 42 
3x27-12 = — = 69 
4x27-12 = — = 96 
5 x 27 - 12 = Yt= 123 
6x27-12 = In = 150 
7 x 27-12= E =177 
8 x 27- 12 = 11 =204 
9 x 27 - 12 = Th = 231 
