NOTES 169 



The Constitution of the Atom 



The discussion at the Royal Society on the " Constitution 

 of the Atom" on March 19 was notable in disclosing real 

 experimental advances of very far-reaching consequences in 

 the most fundamental of all problems, the ultimate structure 

 of matter. Sir Ernest Rutherford, in opening the proceedings, 

 reviewed briefly the chief new methods of attacking the problem 

 — the transformation of one atom into another in radioactive 

 changes, the phenomena observed when high-speed helium 

 atoms or electrons, such as the a- and /3-rays respectively, 

 transverse matter and either penetrate it or are scattered and 

 reflected by it, and, lastly, the regular reflection of the X-rays 

 by crystals. In addition to and distinct from the extraordinary 

 advance in our knowledge of the structure of the molecular or 

 crystalline unit of solids, the last method, by enabling the wave- 

 lengths of the so-called characteristic X-rays to be determined, 

 has thrown an entirely new light on the structure of the atoms 

 of which these X-rays are characteristic. 



The well-known model atom of Sir J. J. Thomson, in 

 which negative electrons are supposed to be distributed through 

 a uniform sphere of positive electricity, cannot give a per- 

 manently stable atom, owing to the continual radiation of 

 energy from the revolving electrons, and, moreover, it is unable 

 to explain the facts met with in the scattering of a-particles. 

 The deflection of a very small proportion of the a-particles 

 through large angles by single encounters with heavy atoms 

 of matter necessitated the existence in these atoms of a very 

 concentrated charge, large in amount but small in volume. 

 This led Sir Ernest Rutherford to picture the atom as com- 

 posed of a nuclear positive charge, excessively small compared 

 with the sphere of action of the atom, and consisting of a 

 number of unit charges equal to about half the atomic mass. 

 A similar number of separate negative electrons are distributed 

 in the external shell, the precise nature of the distribution 

 being still an open question. This model satisfactorily explains 

 the observed laws of scattering of a-particles over the immense 

 range of conditions for which it has been tested. Recently 

 Marsden has studied the passage of a-particles through 

 hydrogen, and has found that some of the hydrogen atoms 

 acquire a velocity by collision so great as to be capable of 

 producing weak scintillations over four times the distance 



