THE CONSTITUTION OF MATTER 133 



of electrons exterior to the nucleus. Several methods of attack on this 

 problem have been suggested. Sir J. J. Thomson showed that the 

 scattering of Eontgen rays in passing through the atoms of matter 

 must depend on the number of electrons composing the atom. By as- 

 suming that each electron scattered is an independent unit, an expres- 

 sion for the scattering was found in terms of the number of electrons 

 in the atom. By comparison of the theory with experiment, Barkla 

 deduced that for many elements the number of electrons in an atom 

 was approximately proportional to its atomic weight and numerically 

 equal to about one half of the atomic weight in terms of hydrogen. 



The charge in the nucleus can also be directly determined from the 

 experiments on scattering of alpha rays, to which attention has pre- 

 viously been drawn. Geiger and Marsden found that the large angle 

 scattering of alpha rays in passing through different substances was 

 proportional per atom to the square of its atomic weight. This showed 

 that the positive charge on the nucleus was approximately proportional 

 to the atomic weight at any rate for elements of atomic weight varying 

 between aluminium and gold. By measuring the fraction of the total 

 number of alpha particles which were deflected through a definite angle 

 in passing through a known thickness of matter, the charge on the 

 nucleus was deduced directly. The number of positive units of charge 

 on the nucleus, which is equal to the number of external negative elec- 

 trons, was found to be expressed by about one half of the atomic 

 weight in terms of hydrogen. The results obtained by two entirely 

 distinct methods of attack are thus in good accord and give approxi- 

 mately the magnitude of this important atomic constant. 



It is obvious, however, that the deduction that the number of units 

 of charge on the nucleus is half the atomic weight, must be only a 

 first approximation to the truth even in the case of the heavier atoms. 

 It has already been pointed out that the nucleus of the helium atom of 

 atomic mass four must carry two unit charges, for it is difficult to be- 

 lieve that any of the exterior electrons of helium can remain attached 

 after its violent expulsion from the atom and its subsequent passage 

 through matter. If this be the^ease, the nucleus of the hydrogen atom 

 of atomic mass one, must carry one unit charge. Van den Broek and 

 Bohr have suggested that the charge on the nucleus might be equal to 

 the actual number of the element when all the known elements are ar- 

 ranged in order of increasing atomic weight. This is in excellent ac- 

 cord with the experiments of scattering and removes a difficulty in 

 regard to the lighter atoms. Taking this view, the nucleus charge is 

 for hydrogen 1, helium 2, lithium 3, carbon 6, oxygen 8, etc. The sim- 

 plicity of this conception has much to commend it. 



During the last year a new and powerful method of attack on this 

 fundamental problem has been developed by Moseley by the study of 

 X-ray spectra. In 1912, Laue found that X rays showed obvious in- 



