November 10, 1916] 



SCIENCE 



661 



particle that the centers of their nuclei 

 must approach within a distance of 

 1.7 X 10" 13 centimeters. This value then 

 would represent a maximum for the sum 

 of the radii of the nuclei of the hydrogen 

 and helium atoms. Rutherford has sug- 

 gested that the nucleus of the hydrogen 

 atom may be the long sought positive elec- 

 tron, and that its dimensions may be con- 

 siderably smaller than half of the maximum 

 dimensions given above. 



The small dimensions of the nucleus 

 offer a possible explanation of the fact that 

 most of the mass is concentrated in the 

 nucleus, provided the mass is electromag- 

 netic in its nature. As already observed 

 the electromagnetic mass of a body is 

 %(e 2 /a), where e is the charge and a the 

 radius. According to this formula the 

 mass increases as the radius decreases. If 

 the mass of the hydrogen atom is to be ex- 

 plained on this basis, the radius of the 

 nucleus must be about %soo that of the 

 negative electron. Rutherford suggests 

 that there is no experimental evidence con- 

 trary to such a view, and that its simplicity 

 has much to commend it. 



Assuming the atom to have a structure 

 similar to that suggested by Rutherford, 

 the determination of the nuclear charge or, 

 what amounts to the same thing, the num- 

 ber of external negative electrons becomes 

 an important matter. Geiger and Marsden 

 calculated the nuclear charge from the 

 number of alpha particles deflected through 

 a definite angle by metallic films of known 

 thickness, and found it to be approximately 

 equal to one half the atomic weight times 

 the charge of an electron. Barkla 12 in 1911 

 experimented on the scattering of X-rays, 

 and determined the number of electrons in 

 a known quantity of matter. These experi- 

 ments were based on the theory of J. J. 

 Thomson that each electron scatters X-rays 



12 Phil. Mag., 21, p. 648, 1911. 



independently, and that an expression for 

 the scattering can be given in terms of the 

 number of electrons. In this way the num- 

 ber of electrons in the atoms of several ele- 

 ments was found to be approximately equal 

 to one half the atomic weight in terms of 

 hydrogen. 



Various lines of investigation indicate 

 that the number of external negative elec- 

 trons, and hence the magnitude of the nu- 

 clear charge, is approximately equal to one 

 half the atomic weight in terms of hydro- 

 gen. In the case of hydrogen, however, it 

 is evident that the number of electrons can 

 not be equal to one half the atomic weight. 

 This has led to an important suggestion by 

 van den Broek, 13 that the number of unit 

 charges on the nucleus, and consequently 

 the number of external negative electrons 

 in any atom may be equal to the number 

 of the corresponding element, when the 

 elements are arranged in the order of in- 

 creasing atomic weights. For example, 

 hydrogen, the first element, would have one 

 electron and one unit charge on the nu- 

 cleus,- helium, the second element, would 

 have two electrons and two charges on the 

 nucleus; carbon, the sixth element, would 

 have six electrons and six charges on the 

 nucleus, and so on. This number is known 

 as the atomic number, and has become an 

 important constant in chemistry. 



Among the most important experiments 

 bearing on the subject of atomic numbers 

 are those of Moseley 14 on the "High Fre- 

 quency Spectra of the Elements." The 

 interference phenomena of X-rays when 

 reflected from a crystal surface have made 

 it possible to determine the wave-lengths 

 and vibration frequencies of these rays. 

 This subject has been especially investi- 

 gated by W. H. and W. L. Bragg. There 

 are at least two kinds of X-rays, the K or 



is Phys. Zeit., 14, p. 33, 1913. 



"PM. Mag., 26, p. 1024, 1913; 27, p. 703, 1914. 



