1922] Isotopes 121 



H3, which was discovered by Thomson, confirmed by Aston, and di- 

 rectly prepared by the activation of H by Wendt and Landauer 

 (Amer. Chem. Soc. 1920. 42: 920). Thus we have the two with the 

 same mass — one an isotope of helium and the other a molecular aggre- 

 gation of hydrogen. They are entirely different in properties. 



Soddy (Lond. Chem. Soc. Ann. Rep. XVII. 221) writes as fol- 

 lows concerning the Aston methods : 



The methods depend on the same general principles as those (of Thomson) 

 which sufficed to detect the presence of meta-neon, of atomic mass 22, in 

 atmospheric neon but the electromagnetic and electrostatic deviating fields are 

 rearranged in such a way as to secure an effect precisely analogous to focus- 

 sing in optics. The trajectories of the positive ions in a slightly divergent beam 

 are brought to a focus in a plane containing the photographic plate. All those 

 for which the mass divided by the charge is the same are brought to the same 

 point in the plane, those with greater or less values, respectively, being on 

 either side. The complex pencil is resolved into a ' ' mass spectrum ' ' in every 

 respect analogous to a light spectrum produced by a prism or grating. The terms 

 "first-order and second-order mass spectrum" are used to denote spectra pro- 

 duced by ions singly and doubly charged respectively. The existence of ions 

 with more than one unit of charge introduces a complication but fortunately 

 these are experimental peculiarities which enable the two orders usually to be 

 distinguished without uncertainty. The relative mass of the ion causing any line 

 in the spectrum can so be evaluated to an accuracy of one part in a thousand 

 and the atomic mass determined to a degree of accuracy comparable with that 

 attained in the best determinations of the atomic Aveight by chemical means. 

 Incidentally, the complete agreement between the two in many cases affords 

 much the most important evidence of the constancy between mass and weight 

 for different elements 



Apart from a possible uncertainty, already alluded to, as to the orders of 

 spectrum to which any line belongs, the photographs published reveal the great 

 power and accuracy of the new method. Unfortunately, only non-metallic ele- 

 ments have so far been included. The difficulties in the way of examining 

 metallic elements by this means have not yet been overcome. In every case, 

 except hydrogen, the atomic mass of each homogeneous component proves to be 

 an exact integer in, terms of that of oxygen as 16, within the error of meas- 

 urement already stated. For hydrogen, however, the chemical value 1.008 is ex- 

 actly confirmed and its homogeneity proved. 



In the year or two that have elapsed since the above citation was 

 written by Soddy several new isotopic elements have been discovered, 

 among them those accompanying certain metals as K, Rb, ]\Ig, and Zn, 

 so some of the difficulties which impeded progress have been overcome. 

 Likewise certain of the atomic weights as determined by chemical 

 means have been very exactly confirmed. Furthermore, Harkins has 



