294 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 13 



nickel would appear among the first ten elements, but such a change 

 will not affect our argument. Ten or eleven elements, all below 

 59 in atomic weight, exceed in abundance all the others. They are 

 structurally among the simplest elements, and therefore, presumably 

 the most stable. 



These conclusions may now be applied to the hypothesis of evolu- 

 tion. The total amount of matter in the original nebula was of 

 course finite; a large part of it was absorbed in building the simpler 

 and more stable elements, and only what remained was available 

 for the development of all the others. This conclusion, I admit, 

 is largely speculative, but it is a legitimate interpretation of evidence. 

 It may be modified by future investigations, but it is not likely to 

 be completely overthrown. It is possible that the relative abundance 

 of the elements may be different in different parts of the solar system, 

 but it is not probable that any of the higher elements can find a 

 place among the first ten. The whole scheme of evolution may be 

 figured diagrammatically as a series of waves in which the crests repre- 

 sent the elements, and the depressions the gaps between them. In 

 such a series the waves would reach their greatest height at iron, and 

 then gradually flatten until the end where instability becomes most 

 clearly evident. 



Between the evolution of the elements and their degradation 

 there is a sharp contrast. The two processes do not follow the same 

 path. Uranium does not decay to thorium, that to radium, then 

 to lead, and so on down the line. The same divergence is shown 

 between the synthesis and decomposition of compounds. It would 

 be easy, for example, to effect a direct synthesis of calcium carbonate 

 from its elements ; but to reverse the process without the intervention 

 of other substances would be extremely difficult. To cite a different 

 example, trinitrotoluene, the T.N.T. of recent warfare, is prepared 

 by the action of nitric acid on toluene, a relatively slow operation. 

 On the other hand, when T.N.T. decomposes it does so instantaneously, 

 and the products are oxides of carbon, methane, water, and free 

 nitrogen. Something like this happens in the decay of a radioactive 

 element, but with a difference; uranium, thorium or radium decom- 

 poses atom by atom; T.N.T. flies to pieces in mass. The one process 

 is slow, the other extremely rapid. 



I have already specified the external conditions which determine 

 the stability of an element or compound, but when we consider the 

 atom by itself, internal conditions are more important. On the 



