THE BIRTH-TIME OF THE WORLD 53 



difficulties in a simpler way, which may be stated as follows : 

 If we assume that all 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 atoms of lead and light atoms of uranium gene- 

 rating 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 is genetically connected with the 

 longer-lived and largely preponderating constituent. There 

 does not seem to be any direct proof of this as yet, however. 



From these considerations it would seem that unless the 

 atomic weight of lead in uraninites, etc, is sub-normal, the 

 former complexity and more accelerated decay of uranium 

 are involved in the data respecting the atomic weights of 

 radium and lead and the radioactive events which occur in 

 the transmutation of the one into the other. As an alternative 

 view, we may assume, as in our first hypothesis, that some 

 elementally different but genetically connected substance, de- 

 caying 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 we are con- 

 fronted 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 radio- 

 active 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. What- 

 ever 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. 



