Chemistry and Physics. 301 



esting as showing how widely the new interpretations of atomic 

 structure derived from physical experimentation have replaced 

 the earlier conclusions obtained from chemical phenomena alone. 

 The term isotype, i. e. same place (in the periodic table) was 

 coined by Soddy in 1913 to designate two elements having differ- 

 ent atomic weights but with chemical properties identical, or so 

 closely resembling each other that they have not yet been separ- 

 ated by chemical methods. In contrast to isotypes, the term 

 isobar is used to designate substances of the same atomic, or 

 molecular weight, but having different chemical properties. Lin- 

 deman thinks it doubtful that the properties of isotypes, though 

 indubitably very similar, are exactly identical and hopes to 

 separate them electrolytically if, as seems likely, they have differ- 

 ent migration velocities in solution. 



On the electron theory the atoms of the isotopes of an element 

 contain an equal number of electrons and the difference in the 

 atomic weight is supposed to be due to the simultaneous entry 

 into the core of the atom, of one or inore positive charges and a 

 corresponding number of electrons so that the charge on the core 

 is not affected. Thus one can suppose that an elementary atom 

 of mass m may be changed to one of mass m -\- lloj the addition 

 of a positive particle and an electron. If both enter the nucleus 

 an isotope results, for the nuclear charge remains unaltered. If 

 only the positive particle enters the nucleus an atom of the next 

 higher atomic number results. In cases where both forms of 

 addition give a stable configuration the new elements will be 

 isobars, i. e. will possess the same atomic weights but have differ- 

 ent chemical properties which are believed to depend upon the 

 number of electrons in the outer layer and its general distance 

 from the center. 



The work of Aston upon the determination of isotopes by the 

 positive ray spectrograph is characterized by Soddy as one of the 

 most brilliant combinations of mathematical analysis and experi- 

 mental skill this century has produced. The method of focusing 



positive rays of constant ^ independently of their velocity and 



producing a "mass spectrum" has been explained in detail by 

 the author in the Philosophical Magazine, 38, 709, 1919, and 39, 

 611, 1920. Briefly it consists in dispersing a ribbon-like beam 

 of positive rays first by an electric field and then recombining 

 a restricted portion of them by a magnetic field so that they 

 produce a definite linear spot or image on a photographic plate. 

 Where several carriers are present the separation of these lines 

 permits their relative masses to be determined to an accuracy 

 of one part of a thousand. The intensity of the lines also per- 

 mits some estimate of the number of each kind to be made. In 

 this way it has been possible to show that some elementary gases 

 consist of a mixture of two or more isotopes. Neon, for example, 



