1922] The Search for the Ultimate Atom 41 



positive electronic charges on the nucleus equals approximately half 

 the atomic weight. This was the first experimental determination of 

 atomic numbers. The rigorous and exact determination of atomic 

 numbers, however, is the work of Mosley. Millikan has character- 

 ized his work as one of the most notable pieces of research in the last 

 fifty years. In his work JMosley made use of X-rays and the crystal 

 grating. 



Barkla had previously discovered that when different elements 

 were used as the anti-cathode each gave off X-rays characteristic of 

 the element used. The Braggs had shown that crystals, because of 

 the regular spacing of their molecules, could be used as reflection grat- 

 ings for X-rays, and that by knowing the distance between the mole- 

 cular planes and the inclination of the rays to the planes the wave- 

 lengths of the rays could be determined. 



Beginning with Al and using the K rays IMosley examined the 

 X-ray spectra of most of the heavier elements. He found that in 

 passing from the lighter to the heavier elements there was a pro- 

 gressive shortening of their wave-lengths, or an increase in their fre- 

 quencies. The functional relation established by Mosley between fre- 

 quency and atomic number is accurate to the degree that when the 

 X-ray spectrum of any element is known its atomic number can 

 be determined unequivocally. The work of Mosley has been extended 

 by De Broglie and others with modifications of the Mosley method. 

 While no X-ray spectra for the ten elements below Na have yet been 

 obtained the properties of these elements are so well known we may 

 confidently rely on the correctness of the numbers assigned them. 



Since the table starts with hydrogen, 1, and ends with uranium, 

 92, with six blanks, it seems that the total number of elements is 

 92 and that six of these are }■ et to be discovered. 



Not only did Mosley 's work substitute a scientifically determined 

 table for the empirical table of Mendeleef, but he proved for the 

 first time that the physical and chemical properties of an element 

 depend upon the nuclear charge. The logical inference from his 

 work appears to be that each element differs from the next lower 

 by the addition of a definite amount of electricity to the nucleus and 

 that this addition causes a corresponding increase in the frequency 

 of the characteristic radiation. 



As the result of the experimental determinations of Thomson and 

 others the hydrogen ion is considered to be the proton, or positive 



