192 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



should hold together, but it must be remembered that we have no 

 information as to the nature or magnitude of the forces existing at 

 such minute distances as are involved in the structure of the nucleus. 



We have so far assumed without proof that while the nucleus of 

 an atom carries a resultant positive charge, negative electrons ar<* 

 also present. The main evidence on this point comes from a study 

 of the radioactive elements. A substance which breaks up with tl>^ 

 emission of swift electrons (beta rays), but no alpha particles, suffers 

 disintegration according to the same hiAvs and gives rise to a new 

 element in the same way as when an alpha particle is lost. It seems 

 necessary to suppose from a number of lines of evidence that a 

 transformation which is accompanied by the emission of primary 

 beta particles must have its origin in the ejection of a negative elec- 

 tron from the nucleus itself or from a point very close to the nucleus. 



There are no means at present of deciding definitely the relative 

 number of positive and negative units composing the nucleus, except 

 possibly from a consideration of the atomic weight of the atom in 

 terms of hydrogen. It is, however, premature to discuss such ques- 

 tions until more information is obtained as to the structure of the 

 nucleus and the effect of concentration and distribution of the com- 

 ponent electrical charges on its apparent mass. 



CHARGE CARRIED BY THE NUCLEUS. 



We are now in a position to consider a very important question, 

 viz, the magnitude of the positive charge carried by the atomic 

 nucleus. Since an atom is electrically neutral, the negative charge 

 carried by the exterior distribution of electrons in the structure of 

 the atom must be equal and opposite to the resultant positive charge 

 carried by the nucleus. The electrical charge is most conveniently 

 expressed in terms of the number of the fundamental units of 

 charge in the nucleus. Since the charge carried by the electron 

 is one unit, the charge on the nucleus of the atom may be expressed 

 numerically by the number 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 Rontgen rays in 

 passing through the atoms of matter must depend on the number 

 of electrons composing the atom. By assuming that each electron 

 scattered is an independent unit, an expression for the scattering 

 was foimd 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 approxi- 

 mately proportional to its atomic weight and numerically equal to 

 about one-half of the atomic weight in terms of hydrogen. 



