We shall now briefly review the results on the geological age of
the Earth.
By methods based on the approximate uniformity of denudative
effects in the past, a period of the order of 100 millions of
years has been obtained as the duration of our geological age;
and consistently whether we accept for measurement the sediments
or the dissolved sodium. We can give reasons why these
measurements might afford too great an age, but we can find
absolutely no good reason why they should give one much too low.
By measuring radioactive products ages have been found which,
while they vary widely among themselves, yet claim to possess
accuracy in their superior limits, and exceed those derived from
denudation from nine to fourteen times.
In this difficulty let us consider the claims of the radioactive
method in any of its forms. In order to be trustworthy it must be
true; (1) that the rate of transformation now shown by the parent
substance has obtained throughout the entire past, and (2) that
there were no other radioactive substances, either now or
formerly existing, except uranium, which gave rise to lead. As
regards methods based on the production of helium, what we have
to say will largely apply to it also. If some unknown source of
these elements exists we, of course, on our assumption
overestimate the age.
23
As regards the first point: In ascribing a constant rate of
change to the parent substance--which Becker (loc. cit.) describes
as "a simple though tremendous extrapolation"--we reason upon
analogy with the constant rate of decay observed in the derived
radioactive bodies. If uranium and thorium are really primary
elements, however, the analogy relied on may be misleading; at
least, it is obviously incomplete. It is incomplete in a
particular which may be very important: the mode of origin of
these parent bodies--whatever it may have been--is different to
that of the secondary elements with which we compare them. A
convergence in their rate of transformation is not impossible, or
even improbable, so far as we known.
As regards the second point: It is assumed that uranium alone of
the elements in radioactive minerals is ultimately transformed to
lead by radioactive changes. We must consider this assumption.
Recent advances in the chemistry of the radioactive elements has
brought out evidence that all three lines of radioactive descent
known to us--_i.e._ those beginning with uranium, with thorium,
and with actinium--alike converge to lead.[1] There are
difficulties in the way of believing that all the lead-like atoms
so produced ("isotopes" of lead, as Soddy proposes to call them)
actually remain as stable lead in the minerals. For one
[1] See Soddy's _Chemistry of the Radioactive Elements_ (Longmans,
Green & Co.).
24
thing there is sometimes, along with very large amounts of
thorium, an almost entire absence of lead in thorianites and
thorites. And in some urano--thorites the lead may be noticed to
follow the uranium in approximate proportionality,
notwithstanding the presence of large amounts of thorium.[1] This
is in favour of the assumption that all the lead present is
derived from the uranium. The actinium is present in negligibly
small amounts.
On the other hand, there is evidence arising from the atomic
weight of lead which seems to involve some other parent than
uranium. Soddy, in the work referred to, points this out. The
atomic weight of radium is well known, and uranium in its descent
has to change to this element. The loss of mass between radium
and uranium-derived lead can be accurately estimated by the
number of alpha rays given off. From this we get the atomic
weight of uranium-derived lead as closely 206. Now the best
determinations of the atomic weight of normal lead assign to this
element an atomic weight of closely
[1] It seems very difficult at present to suggest an end product
for thorium, unless we assume that, by loss of electrons, thorium
E, or thorium-lead, reverts to a substance chemically identical
with thorium itself. Such a change--whether considered from the
point of view of the periodic law or of the radioactive theory
would involve many interesting consequences. It is, of course,
quite possible that the nature of the conditions attending the
deposition of the uranium ores, many of which are comparatively
recent, are responsible for the difficulties observed. The
thorium and uranium ores are, again, specially prone to
alteration.
25
207. By a somewhat similar calculation it is deduced that
thorium-derived lead would possess the atomic weight of 208. Thus
normal lead might be an admixture of uranium- and thorium-derived
lead. However, as we have seen, the view that thorium gives rise
to stable lead is beset with some difficulties.
If we are going upon reliable facts and figures, we must, then,
assume: (a) That some other element than uranium, and genetically
connected with it (probably as parent substance), gives rise, or
formerly gave rise, to lead of heavier atomic weight than normal
lead. It may be observed respecting this theory that there is
some support for the view that a parent substance both to uranium
and thorium has existed or possibly exists. The evidence is found
in the proportionality frequently observed between the amounts of
thorium and uranium in the primary rocks.[1] Or: (b) We may meet
the difficulties in a simpler way, which may be stated as
follows: If we assume that all stable lead is derived from
uranium, and at the same time recognise that lead is not
perfectly homogeneous in atomic weight, we must, of necessity,
ascribe to uranium a similar want of homogeneity; heavy atoms of
uranium giving rise to heavy
[1] Compare results for the thorium content of such rocks
(appearing in a paper by the author Cong. Int. _de Radiologie et
d'Electricité_, vol. i., 1910, p. 373), and those for the radium
content, as collected in _Phil. Mag._, October, 1912, p. 697.
Also A. L. Fletcher, _Phil. Mag._, July, 1910; January, 1911, and
June, 1911. J. H. J. Poole, _Phil. Mag._, April, 1915
26
atoms of lead and light atoms of uranium generating light atoms
of lead. This assumption seems to be involved in the figures
upon, which we are going. Still relying on these figures, we
find, however, that existing uranium cannot give rise to lead of
normal atomic weight. We can only conclude that the heavier atoms
of uranium have decayed more rapidly than the lighter ones. In
this connection it is of interest to note the complexity of
uranium as recently established by Geiger, although in this case
it is assumed that the shorter-lived isotope bears the relation
of offspring to the longer-lived and largely preponderating
constituent. However, there does not seem to be any direct proof
of this as yet.
From these considerations it would seem that unless the atomic
weight of lead in uraninites, etc., is 206, the former complexity
and more accelerated decay of uranium are indicated in the data
respecting the atomic weights of radium and lead[1]. As an
alternative view, we may assume, as in our first hypothesis, that
some elementally different but genetically connected substance,
decaying along branching lines of descent at a rate sufficient to
practically remove the whole of it during geological time,
formerly existed. Whichever hypothesis we adopt
[1] Later investigation has shown that the atomic weight of lead
in uranium-bearing ores is about 206.6 (see Richards and Lembert,
_Journ. of Am. Claem. Soc._, July, 1914). This result gives support
to the view expressed above.
27
we are confronted by probabilities which invalidate
time-measurements based on the lead and helium ratio in minerals.
We have, in short, grave reason to question the measure of
uniformitarianism postulated in finding the age by any of the
known radioactive methods.
That we have much to learn respecting our assumptions, whether we
pursue the geological or the radioactive methods of approaching
the age of our era, is, indeed, probable. Whatever the issue it
is certain that the reconciling facts will leave us with much
more light than we at present possess either as respects the
Earth's history or the history of the radioactive elements. With
this necessary admission we leave our study of the Birth-Time of
the World.
It has led us a long way from Lucretius. We do not ask if other
Iliads have perished; or if poets before Homer have vainly sung,
becoming a prey to all-consuming time. We move in a greater
history, the landmarks of which are not the birth and death of
kings and poets, but of species, genera, orders. And we set out
these organic events not according to the passing generations of
man, but over scores or hundreds of millions of years.
How much Lucretius has lost, and how much we have gained, is
bound up with the question of the intrinsic value of knowledge
and great ideas. Let us appraise knowledge as we would the
Homeric poems, as some-
28
thing which ennobles life and makes it happier. Well, then, we
are, as I think, in possession today of some of those lost Iliads
and Odysseys for which Lucretius looked in vain.[1]
[1] The duration in the past of Solar heat is necessarily bound
up with the geological age. There is no known means (outside
speculative science) of accounting for more than about 30 million
years of the existing solar temperature in the past. In this
direction the age seems certainly limited to 100 million years.
See a review of the question by Dr. Lindemann in Nature, April
5th, 1915.
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