120 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 34 



Early in 1929 the complexity of oxygen was established by Giau- 

 que and Johnston of California, using a novel method of attack, by 

 examining the absorption of light by air. They found absorption 

 bands which were interpreted as belonging to compounds containing 

 two new oxygen isotopes, one of mass 18 and a much rarer one of 

 mass 17. Oxygen, of mass 16, had been used as the standard of mass 

 reference for all the other elements both for historical reasons and 

 because of its assumed simplicity. Its established complexity at 

 once raised doubts as to the simplicity of carbon and hydrogen. In 

 the case of the former, the doubts were resolved by the discovery, 

 in 1929, of a rare isotope of mass 13 by Birge and King, again from 

 a study of the band spectra of gaseous carbon compounds, among 

 others that of carbon monoxide. Birge and Menzel calculated that 

 discrepancies between the chemical atomic weight and the mass spec- 

 trograph value for hydrogen would be resolved if hydrogen contained 

 about one part in 4,500 of an isotope of mass 2. It was this theoretical 

 calculation which provided the spur for an experimental search for 

 such an isotope by Urey, Brickwedde, and Murphy, jointly, at Co- 

 lumbia University, and the United States Bureau of Standards. 

 They announced early in 1932 that, by fractional distillation of liq- 

 uid hydrogen, the heavier isotope concentrated in the residue, and 

 that its presence could be demonstrated by the appearance of a 

 faint spectral line in the hydrogen discharge near the ordinary line 

 of atomic hydrogen and spaced from it at such a distance as would 

 be demanded theoretically for an atom with a charge of unity (that 

 is to say a hydrogen isotope), but having a mass of 2. 



Atomic weight determinations, mass spectrogi'aphic and light 

 absorption measurements only demonstrate the existence, the rela- 

 tive abundance and the masses of isotopes. The practical identity 

 of their chemical properties, emphasized at the outset by Soddy, had 

 been utilized in the case of radioactive isotopes for chemical indicator 

 purposes; the desirable goal of the scientist, the separation of the 

 isotopes of an element and the separate examination and comparison 

 of their properties, remained until a year ago unattained. An enor- 

 mous amount of effort has been expended in the attempts at sepa- 

 ration. These must be based on differences in properties which 

 depend essentially on mass or on chemical reactivity. For a decade 

 and a half prior to 1933 a variety of trials were made. Separation 

 was attempted by fractional diffusion, by thermal diffusion, by cen- 

 trifugal separation, by fractional distillation and evaporation at low 

 pressure, by migration of isotopic ions under the influence of an 

 electric current, by preferential excitation to photochemical reaction 

 of one or other isotope using light absorbed by one and not the other. 



