SOMF. COXTEAfPOR.IRV .IPr.lNCF.S l\ /V/r.T/CT -If ASf) 



charges, <if the vaeaiil spare armiiul them; and if the iHTceiitaRe of 

 scattered ()articles is measured absolutely, the absolute value of the 

 charge of the nucleus can Ix; calculated. These values have actually 

 been obtainetl : " 



Platinum: nuclear charge (77.4±n<' (Chadwick) 

 Silver; (4«.3±0.7)e (Chadwick) 



Copper; (29.3±0.5)f (Chadwick) 



Argon: 19e (Auger and Perrin) 



"Air" 6.5c (C. T. R. Wilson) 



Bohr's interpretation of the spectra of hydrotien and ionized helium. 

 There is a complete and perfect agreement between the observed 

 frequencies in the spectra of hydrogen and ionized helium, and the 

 frequencies predicted by Bohr. .An essential feature of Bohr's theory 

 is that the charge on the nucleus of the hydrogen atom is assumed 

 to be e, and the charge on the nucleus of the helium atom to be 2e. 

 .As there is no other element of which the spectrum has been perfectly 

 and completely explained by Bohr's theory (or any other) this affirma- 

 tion cannot be extended be%ond hydrogen and helium. 



Thus we have excellent evidence from three distinct .sources that the 

 nuclear charge of helium, the second element of the periodic table, 

 is 2e; excellent evidence from two sources that the nuclear charge of 

 the first element, hydrogen, is e\ and good evidence by the alpha-ray 

 method that the nuclear charges of the 18th, 29th, 40th and 78th 

 element are as close to 18e, 29f, 4t)f, and 78f as to any other integer 

 multiple of e. In addition, there is evidence from two more sources 

 that, in passing from one element to the next along the procession of 

 elements, one finds the nuclear charge augmented by the amount e 

 at each step; thus completing the itemized evidences foregoing by a 

 process somewhat like what is called "mathematical induction." 



The displacement-law of Fajans and Soddy. When an atom-nucleus 

 of a radioactive element disintegrates by shooting off an electron 

 bearing a charge —e, the residuum is found to be a nucleus of an 

 element one step farther up in the procession of elements. When an 

 atom-nucleus disintegrates by shooting off an alpha-particle bearing a 



'"The earliest experiments ^discussed by Rutherford in 1911) demonstrated that 

 for several metals the nuclear charge (mcasurctl in terms of e) was alH)ut one-half 

 the atomic weight; and those of Geiger and Marsden ( IQl.?) were arranged primarily 

 to <lenion4trate the validity of the concept of the nuclear atom, but confirmeil that 

 statement for gold. Chadwick repeated these experiments U[>on Pi, .Ig and tu 

 with the object of determining the nuclear charge as accurately as possible. The 

 values for argon and "air" were determined by what is in principle the same method 

 though in a very different form; the former with alpha-particles, the latter with 

 fast electrons. 



