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CHEMISTRY: F. W. CLARKE 
and replaced by a multitude of compounds. On this basis hydrogen and 
helium seem to be the oldest of the known elements, while uranium and thorium 
are the youngest of all. Lead is older than uranium and thorium, for its 
lines appear in the solar spectrum, in which the other two elements have 
not as yet been recognized. Lead, however, is vastly more abundant than 
either uranium or thorium, and is more likely to have been originally their 
progenitor than their child. 
Up to this point we have a reasonable interpretation of definite evidence, 
beyond this, imagination must come into play. It is fair to assume that 
the process of evolution was extremely slow, and that each element was de- 
veloped gradually and passed from an unfinished to a finished stage. The 
chemical atoms are now known to be extremely complex structures, each 
with an electropositive nucleus surrounded by electrons in rapid motion. 
That such a structure could have been developed instantaneously, with no 
previous preparation, is hardly probable, for the process was one of con- 
densation, from lighter to heavier, and that, it would seem, must have ac- 
quired time. The process was one from relative simplicity of structure to 
relative complexity, and with the maximum condensation, as shown by 
uranium and thorium, a minimum of stability was reached. That is, so far as 
we now know; for less stable atoms may have been formed, to exist for a brief 
period and then vanish. Some of the radioactive elements which appear 
as products of the decay of uranium are of this kind. On that theme, more 
later. 
That the atoms of the elements above helium in the scale of atomic weights 
could not have been formed instantaneously is indicated by their structure. 
It has been shown that they are built up of smaller particles, of electrons, 
and also in part, perhaps, of preexistent helium. Such particles, approach- 
ing one another, at first in irregular proportions, are supposed to have formed 
the atoms in question; but that exactly the right proportions for stability 
were found at once is hardly conceivable. There must have been a period 
of selection, in which the unavailable particles were discarded, probably to 
be used in other structures later. For each new chemical atom a definite 
balance between electropositive and electronegative particles was required, 
and also the establishment of a stable configuration. When these condi- 
tions were fulfilled the atom of an element was complete. As I have already 
said we can fairly assume that there was a distinct passage from an unfinished 
or incipient structure to a finished one of permanent stability. Further- 
more, as shown by the spectra of stars and nebulae, the elements of relatively 
low atomic weight were first formed, and those of higher atomic weight came 
later. The older elements were also developed in the largest quantities, and 
are therefore the most abundant. The later elements are as a broad general 
rule much scarcer. This rule is not absolutely exact, but it expresses some 
well known general relations. The very simple and very stable primordial 
