50 Messrs. A. C. and A. E. Jessup on the 
of digits from the second term to the third is eleven, which 
is the number of elements to which silicon gives rise. 
With reference to the rise and fall which we have developed 
in our mathematical equation as shown above, we would point 
out that if there is any real basis for this phenomenon, it must 
be exhibited in the properties of the elements concerned. 
That there is indeed a very striking connexion is clearly 
shown in the following Table of oxides and chlorides which 
are known to exist for the elements of the nitrogen, oxygen, 
and fluorine groups : — 
BiCL 
l 3 
PC1 3 
AsCl 3 
SbCl 3 
PCL 
— 
SbCL 
5 
SC1 Q 
— 
Te Cl 2 
sci 4 
SeCl 4 
Te Cl 4 
so 2 
Se0 2 
Te0 2 
so 3 
— 
TeO" 3 
CI OH 
Br OH 
I OH 
CIO OH 
— 
ICI3 
CI 2 OH 
Br 2 OH 
I 2 OH 
CI O3OH 
— 
I O^OH 
Moreover, since the members of the first three groups do 
not commonly exert more than one valency, we cannot expect 
to find any phenomenon analogous to the above in their case. 
Also in the silicon family the sub-groups are too small to 
show the same, and consequently any rise and fall in affinity 
as shown by variable valency should only be looked for in the 
nitrogen, oxygen, and fluorine groups, and it is precisely here 
it does occur. From this it appears that the alternate members 
of these groups, viz., nitrogen, arsenic, bismuth, selenium, and 
bromine, do not exert their valencies to such an extent as the 
remaining elements. The pentachlorides of nitrogen, arsenic, 
and bismuth do not exist, and though As 2 5 is known, it is 
much less stable than P 2 5 or Sb 2 5 . Again, selenium is 
only tetravalent in its simple compounds with oxygen and 
chlorine, whereas the other members of its family are divalent, 
tetravalent, and hexavalent as well. The selenates with 
hexavalent selenium exist, but are comparatively unstable. 
Finally, in the halogen family, no compounds of bromine are 
known in which this element is tri- or heptavalent. 
While offering no explanation for this pheuomenon, we 
think that it is at least highly interesting, especially when 
viewed in the light of the rise and fall in atomic weight given 
by our formula. 
We would further point out that the maximum fall occurs 
in the case of the rare earth thulium in the second group. 
