414 SECTIONAL TRANSACTIONS.—A. 
through a series of changes (radium is one of the stages in its progress), 
changing eventually into an isotope of lead—that is, an element chemically 
indistinguishable from lead except a slight difference in atomic weight, and 
inseparable from ordinary lead by chemical means if once mixed with it. The 
isotope in question has probably an atomic weight of 206 exactly, as contrasted 
with 207.1 for ordinary lead, which is doubtless to some extent a mixture of 
isotopes. Thus the product has a much less atomic weight than uranium 
(238.5), and the difference represents approximately the weight of helium 
atoms, which are the debris shed at the various stages of the transformation. 
Further, it is well established that a gram of uranium as found along with 
its products in rocks and minerals is now changing at a rate represented by 
the production of 1.88 x 10-!! grams of helium and 1.22 x 10-!9 grams of lead 
isotope per annum. There is every reason to believe that this is also the rate 
at which 1 gram of uranium has changed in the past, since the rate is un- 
affected by any change of temperature or pressure which we can apply. 
Minerals containing uranium are always found to contain helium and lead. 
The helium may safely be treated as wholly a radioactive product. It would 
be difficult to account for its presence, retained mechanically in the mineral, in 
any other way. The lead in some cases conforms itself to the expected atomic 
weight of 206, about one unit lower than common lead, and in such cases we 
may safely regard the whole of it as a product of uranium disintegration. 
Thus, take the broggerite found in the pre-Cambrian rocks at Moss, Norway. 
The lead in this monad has an atomic weight of 206.06 as determined by 
Ho6nigschmid and Fraulein St. Horovitz. The ratio of lead to uranium is .113. 
Taking the lead as all produced by uranium at the rate above given, we get 
an age of 925 million years. Some minerals from other archean rocks in 
Norway give a rather longer age. A determination of the amount of helium in 
minerals gives an alternative method of estimating time. But helium, unlike 
lead, is liable to leak away, hence the estimate gives a minimum only. I have 
found in this way ages which, speaking generally, are about one-third of the 
values which estimates of lead have given, and are therefore generally confirma- 
tory, having regard to leakage of helium. This method can be applied to 
material found in the younger formations. Thus the helium in an eocene iron 
ore indicated 30 million years at least. 
H. N. Russel has recently applied the argument from accumulation of lead 
to the earth’s crust as a whole. He takes the uranium as 7x10 of the whole, 
the lead as 22x10°% of the whole. If all the lead were uranium lead, and 
had been generated since formation of the crust, the time required would be 
11x10 years. This is certainly too great. Allowing for the production of 
some of the lead by uranium, Russel finds 8x10% years as the upper limit. 
This is about six times the age indicated by the oldest individual radioactive 
minerals that have been examined. 
The upshot is that radioactive methods of estimation indicate a moderate 
multiple of 1,000 million years as the possible and probable duration of the 
earth’s crust as suitable for the habitation of living beings, and that no other 
considerations from the side of physics or astronomy afford any definite pre- 
sumption against this estimate. The argument from geology and biology I 
must leave to our colleagues from other sections. May I venture to say that 
I for one consider the topics with which they will deal as not less interesting 
and important than those which it has been my privilege to lay before you? 
Prof. J. W. Grecory, F.R.S.—The claim that geological time must be 
restricted within a score or a few score million years was regarded by most 
geologists with incredulity, since a score million years was of little more use to 
geology than the seven days of the Pentateuch. Now that physical evidence 
allows the age of the earth to be counted by the thousand million years the 
problem is of less concern to the geologist, except from the hope that the 
uranium-lead ratio may fix geological dates in years, and from the interest of 
reconciling the conflicting results of the different methods. 
The geological estimates to which most weight has been attached are based 
on the saltness of the sea. The salinity argument has been widely accepted 
as sound in principle; the estimates varied from 70 to 150 million years, and 
some intermediate length was regarded as inevitable. Allowances were made 
