CHEMISTRY: F. W. CLARKE 
185 
The latter assumption is now seen to be extremely doubtful for the varying 
atomic weights prove that more than one kind of lead must be considered. 
Thorium lead especially must be taken into account, for many uraninites 
contain it, and in thorianite the percentage of thoria is more than five times 
that of uranium oxide. The ratio of lead to its parent elements is therefore 
much less than Boltwood assumed, and the calculated age of thorianite is 
vastly reduced. Boltwood, however, doubted the derivation of lead from 
thorium, a fact which was not definitely known at the time his paper was 
written. The evidence of the atomic weights is also much later. 
Furthermore, the, doubtful applicability of Boltwood's method to chrono- 
logical measurements has been shown by G. F. Becker; 12 who applied it to 
the analyses of rare-earth minerals from one locality in Llano County, Texas. 
The figures given by Becker are as follows: 
Mineral Analyst Calculated age in years 
Yttrialite Mackintosh 11 ,470,000,000 
Yttrialite Hiliebrand 5,136,000,000 
Mackintoshite Hiliebrand 3,894,000,000 
Nivenite Mackintosh 1,671,000,000 
Fergusonite Mackintosh 10,350,000,000 
Fergusonite Mackintosh 2,967,000,000 
These ages differ enormously, even between two analyses of the same 
mineral. This evidence, taken together with the evidence from the atomic 
weights, seems clearly to show that the uranium-lead ratio is not applicable 
to the determination of the age of minerals. It is quite certain that not all 
of the lead in uranium ores is of radioactive origin. In pitchblende, for ex- 
ample, which contains no thorium, the determinations of atomic weight 
range from 206.40 to 206.88, figures far in excess of the theoretical 206.00 which 
is assigned to pure uranium lead. Normal lead, perhaps in solid solution, 
must be present in such ores. 
What, now, is the fundamental difference between normal lead and isotopic 
lead? The answer to that question must be largely speculative; but specu- 
lation is legitimate when its purpose is to stimulate future research. One 
difference at least may reasonably be assumed, namely, that normal lead is 
the product of an orderly evolution of the chemical elements; and that isotopic 
lead is a product of their decay. Creation is one process, destruction is the 
other. 
Forty-five years ago 13 I ventured to suggest that an evolution of the ele- 
ments had actually occurred. It was clearly indicated by the progressive 
chemical complexity of the heavenly bodies, from the chemically simple 
gaseous nebulae, through the hotter stars and the sun, to the finished planets 
like our earth. At first, hydrogen and helium were the most abundant 
and conspicuous elements, then elements of higher atomic weight gradually 
appeared, and at the end of the process there was the chemical complexity 
of the earth, in which the free elements had in great part been absorbed 
