8GQ Dr. J. W. Nicholson on a Structural 



one, and it is noteworthy that Rutherford *, in a recent 

 paper, has concluded, from the evidence furnished by- 

 experiments on scattering, that the planetary system is 

 the most probable. But although the size of the positive 

 units is so different, Thomson's conception of the uniform 

 volume distribution is essential to the calculations of the 

 present theory. 



In a complex atom, built up of simpler systems, the 

 assemblage of positive charges is in many respects similar 

 to the assemblage of electrons which revolve round them, 

 and it is not unlikely that many of the positive charges 

 would also revolve. But they are not all of the same size, 

 although the difference in size is not great. Their mass is 

 so great that a disturbance which could expel one of them 

 from an atom would also expel many of the attendant 

 electrons, and it would be impossible to isolate a positive 

 charge. Evidence for the existence of such positive electrons 

 should be furnished by certain experimental results ; and 

 this has recently been the case. In particular, a paper by 

 Prof. R. W. Wood t may be mentioned. 



The phenomena of radioactivity afford abundant evidence 

 in the same direction. For example, the radium emanation 

 has been shown, by the work of Ramsay and others, to belong 

 without doubt to the argon group of inert gases, and to be a 

 true element for which a vacant space has existed hitherto in 

 the Periodic Table. Yet it gives off the simpler atom of 

 helium. Unless the constituents of this atom already exist 

 as a group, in unstable equilibrium with other groups, in the 

 atom of the emanation, it is difficult to imagine by what 

 means it can be detached as such when a really definite 

 conception of the process is intended. Evidently the difficulty 

 is extreme on Sir J. J. Thomson's view of the atom, which, 

 it must be borne in mind, was only a provisional one adopted 

 mainly for purposes of mathematical simplicity. It is none 

 the less extreme on Rutherford's more recent view, a revival 

 of the suggestion of Nagaoka, of a simple Saturnian system 

 in the atom, involving only a single positive nucleus. 



The mode of analysis adopted in this paper has neverthe- 

 less, by way of test, been applied to such a system ; but it 

 has been found impossible from such a conception to obtain 

 atomic weights in good accord, or in some cases in any accord 

 at all, with chemical determinations for the simpler elements. 

 Similar application has been made to other conceptions of 

 positive electricity — for example, surface distributions of 

 uniform density, — and with the same lack of success. On 

 * Phil. Mag. May 1911. f Phil. Mag. Feb. 1908. 



