ARTICLES 221 



those already exhibiting it makes the probabihty of this ex- 

 tremely high. The idea put forward by Prout, that all ele- 

 ments were built of atoms of a common substance which he 

 called " Protyle," is therefore to a certain extent substanti- 

 ated. The atom of protyle regarded according to modern 

 views is a neutral pair of oppositely charged atoms of elec- 

 tricity closely combined, as in the nucleus of helium or that 

 of oxygen, not comparatively widely separated as in the atom 

 of hydrogen. The fact that the positively charged atom of 

 hydrogen — which must be the positive atom of electricity 

 itself — differs from unity to a measurable extent is probably 

 the result of its not being associated with a negative charge, 

 but free to exercise its maximum mass effect. 



General Considerations and Conclusions. — The Rutherford- 

 Bohr atom model consists of a central nucleus around which 

 rotate planetary electrons. The mass of an electron is 0-00054 

 on our scale, so that the whole mass of the atom can be re- 

 garded as residing in the nucleus. The number of planetary 

 electrons on a neutral atom must, of course, be equal to the 

 number of excess positive charges on the nucleus, which is 

 most probably Moseley's atomic " number." All the chemical 

 and spectroscopic properties of the atom seem to depend on 

 these planetary electrons, and therefore on the charge on the 

 nucleus, and only to an insignificant extent on its mass. Iso- 

 topes are elements having the same nuclear charge but different 

 nuclear mass. Thus one may now suppose that an elementary 

 atom of mass m may be changed to one of mass m -}- i by the 

 addition of a positive particle and an electron. If both enter 

 the nucleus an isotope results, for the nuclear charge is un- 

 altered. It the positive particle only enters the nucleus, an 

 element of next higher atomic number is formed. In cases 

 where both forms of addition give a stable configuration, the 

 two elements will be " isobares " — that is, elements of equal 

 atomic weight but different chemical properties. 



The only fact which appears to limit the number of isotopes 

 of one element is that, given any number of positive and nega- 

 tive charges with which to build a nucleus having the ne- 

 cessary excess positive charge, only a few of the possible 

 configurations will be stable enough to exist at all. Partially 

 stable arrangements would correspond to radio-active elements. 

 These considerations lead one to expect that the greater the 

 total number of positive and negative charges, the greater the 

 number of stable or partially stable configurations which can 

 be built with them. So that the higher the atomic weight of 

 an element, the more isotopes it may have, and the greater the 

 chance of some of these being radio-active. Both these conclu- 

 sions are in accordance with experience. 



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