224 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1&20. 



hypothesis untenable. Chlorine, for instance, has the atomic weight 

 35.46 and can not be forced to take an integral value. 



Readers of the Smithsonian Reports of 1918, 1915, and 1914 may 

 recall in the articles of Millikan, Rutherford, and Eve references to 

 the inspiring discovery of the so-called " atomic numbers " by Mose- 

 ley, whose brief life of brilliant promise was lost in the ill-fated 

 British expedition to the Dardanelles. Moseley found that the X-ray 

 spectra of the elements had a regular periodicity expressible by the 

 integral numbers progressing by unit steps from element to element 

 nearly in the order of their increasing atomic weights. Certain gaps 

 in the Moseley atomic numbers indicate the existence of a few ele- 

 ments hitherto undiscovered. Thus the irritating deviations from 

 integers of the atomic weight values gave place to a beautiful round- 

 ness in the Moseley system of X-ray spectrum atomic numbers. 



By this time it had become impossible to define an element as a 

 substance incapable of disintegration, for not long after the discovery 

 of radium several " elements," including uranium, radium, and others, 

 were found to be continuous^ disintegrating with the evolution of 

 heat. One product of this degradation was found to be helium and 

 another the metal lead. Thus does nature solve the alchemist's prob- 

 lem of accomplishing a transmutation of the elements. But her 

 methods are not in man's control either to accelerate, retard, or 

 modify the final products. 



We must follow still another thread of investigation before we 

 are ready to take up Doctor Aston's experiments. Prior to Roent- 

 gen's discovery of the X rays in 1895, as described by him in the 

 Smithsonian Report for 1897, a deal of investigation had gone on in 

 relation to the phenomena of the electric discharge in vacuum tubes. 

 It will be convenient to recall the terms used by physicists for the 

 essential parts of the apparatus used in this work. Of the two me- 

 tallic conductors inserted through the glass in a vacuum discharge 

 tube, the negative is called the " cathode," the positive the " an- 

 ode." When under the force of the electric field streams of posi- 

 tively charged particles move through the highly rarefied gas to- 

 ward the cathode, they, upon striking it, cause it to emit at right 

 angles to its surface the so-called " cathode rays." If the cathode is 

 perforated, part of the stream may pass through the holes, produc- 

 ing luminosity behind the cathode, and forming the so-called 

 " canal-strahlen " or " positive rays." 



It now appears that the cathode rays are negatively charged 

 bodies called ions or electrons, having masses less than a thou- 

 sandth part of that of an atom of hydrogen, but the positive rays 

 are composed of positively charged particles ranging from about 

 the mass of the hydrogen atom, as a minimum, to much larger 



