38 Messrs. A. C. and A. E. Jessup on the 
In discussing the position of the four protons, viz., hydrogen, 
proto-bery Ilium, proto-boron, and helium, we placed helium 
at the head of the family of rare gases, saying that it acts as 
the progenitor of this family. Of course it is usually assumed 
from ordinary chemical evidence that these elements have no 
powers of entering into combination, and it might be argued 
from this that it would be impossible for helium to produce 
evolution products of its own type. Unless, however, w r e 
make the somewhat unjustifiable assumption that the elements 
neon, argon, krypton, and xenon are entirely devolution 
products of some radioactive processes, we must assume that 
they possess some form of residual valency. It is quite true 
that no definite evidence of this residual valency has yet been 
proved by our somewhat crude methods of chemical obser- 
vation. There is no doubt in our own minds that this residual 
valency does exist, and we are in great hopes that before very 
long some physical method may be found of determining its 
existence and its amount. 
In connexion with the general scheme of evolution as set 
forth, a most interesting point arises which is well worth 
noticing. We have already stated that the tendency of the 
four electrons on the outer ring of the element silicon to 
complete the system of eight is very small, and that, conse- 
quently, the tendency to form direct derivatives will be 
relatively small. The tendency therefore of the element 
silicon to give rise to indirect processes of evolution will be 
relatively greater ; and in this fact we have an explanation 
of why the indirect derivatives of silicon occur in so much 
greater quantities than the direct derivatives. The indirect 
derivatives to which we especially refer are manganese, iron, 
nickel, cobalt, copper, and zinc. 
There is one most important aspect of any evolution 
process which must not be forgotten. As stated in our pre- 
liminary account on page 23, the whole process of evolution 
must of necessity be governed by the fact that evolution is 
attended by decrease of energy content, for it is perfectly 
clear that this is the fundamental law of physical or chemical 
change. If w r e consider any particular series in the Periodic 
Table, we should expect to find some evidence of this decrease 
of energy content. It is naturally impossible that we should 
be able to express in any way the actual energy content of 
any atom, but we can gain considerable information by 
making comparative measurements along any series. For 
example, if we take the series of the halogens, we find that the 
first member, fluorine, has very powerful chemical properties ; 
that is to say, the desire to pick up electrons to complete the 
