CONSTITUTION OF MATTER EUTHEEFORD. 193 



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 sub- 

 stances was proportional per atom to the square of its atomic weight. 

 This showed that the positive charge on the nucleus was approxi- 

 mately proportional to the atomic w^eight at any rate for elements 

 of atomic weight varying between aluminium and gold. By meas- 

 uring the fraction of the total number of alpha particles which were 

 deflected through a definite angle in passing through a known thick- 

 ness 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 electrons, was found to be expressed 

 by about one-half of the atomic Aveight in terms of hydrogen. The 

 results obtained by two entirely distinct methods of attack are thus 

 in good accord and give approximately the magnitude of this impor- 

 tant 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 believe 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 case, 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 arranged in order of 

 increasing atomic w^eight. This is in excellent accord with the 

 experiments of scattering, and removes a difficulty in regard to the 

 lighter atoms. Taking this view, the nucleus charge is for hydro- 

 gen 1, helium 2, lithium 3, carbon 6, oxygen 8, etc. The simplicity 

 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 

 interference or diffraction effects in their passage through crystals, 

 thus proving definitely that the X rays consist of very short waves 

 analogous to those of light. W. H. Bragg and W. L. Bragg and 

 Moseley and Darwin found that the reflection of the X rays from 

 crystals provided a very simple method of measuring the wave 

 length of the X rays when the spacing of the atoms in the crystal is 

 known. If the X rays give a spectrum containing some bright lines, 

 18618°— SM 1915 13 



