THE TERRESTRIAL DISTRIBUTION OF RADIUM 27 



that owing to the closer packing of the atoms, intense repulsive 

 forces between adjacent nuclei, or between neighbouring groups 

 of electrons, might be setup, and that this would tend to prevent 

 escape of the a- and /3-particles. This seems to be plausible, 

 but until the dimensions of atoms are known with sufficient 

 accuracy to investigate their individual average densities and 

 their mutual spatial relations, it is impossible to decide whether 

 an increase of molar density due to pressure could, within the 

 earth, suffice to induce inhibition. At present it would appear 

 that unless the density of a solid were increased many times by 

 the application of pressure — a condition which is certainly 

 unrepresented in any part of our planet — then the suggested 

 cause would be inadequate to explain the hypothetical pheno- 

 menon attributed to it. 



Experimental evidence, as far as it goes, supports these 

 theoretical considerations, for, although experiments have been 

 made in the hope of detecting some change when radioactive 

 substances are vigorously bombarded with a-, /3-, and 7-rays, no 

 effect has been observed. It would, for example, be reasonable 

 to suppose that when radium emanation is disintegrating, a 

 small amount of radium, the immediate parent of the emanation, 

 might be formed by the re-introduction into the latter of newly 

 liberated a-particles. Rutherford has experimented on these 

 lines with a number of substances, but in no case has evidence 

 been obtained that the process of transformation is reversible. 

 It is, unfortunately, impossible to arrive at a satisfactory con- 

 clusion, but (a) experimental results, (b) theoretical considerations 

 based on the constitution of the atom, the violence of atomic 

 decay, and the incompressibility of solids, and (c) spectroscopic 

 evidence of stellar and atomic evolution, all tend to the final 

 suggestion that atomic disintegration is mainly controlled by 

 conditions other than those of pressure and temperature. 



This necessarily vague conclusion can scarcely form a sound 

 basis on which to build a theory of the terrestrial distribution 

 of radium. It certainly points to the probability that the radio- 

 elements, if they were distributed throughout the substance of 

 the earth, would disintegrate much as they are known to do at 

 the surface, and that, since the earth is not growing hotter, the 

 radio-elements are therefore limited in their occurrence to a 

 comparatively thin superficial shell. In justice to the evidence, 

 no more definite statement than this can be made. We must 



