20 SECTIONAL ADDRESSES. 



Lockyer's hypothesis that such lines originated in dissociated atoms — 

 i.e. atoms split up into parts of comparable size, constituting ' proto- 

 e.lements' — was in fact replaced in the new theory by the conception that 

 the first stage of dissociation is the breaking up of a neutral atom into an 

 ionised atom and an electron. An excited atom in which the series 

 electron does not pass outside the sphere of influence of the nuclear charge 

 remains neutral as a whole, and the spectrum is that of the neutral atom, 

 having the Eydberg number R for the series constant. If the most loosely 

 bound electron be driven out of the atom, and the next most loosely 

 bound one transferred from its normal position to larger orbits by the 

 exciting agency, the spectrum generated by the return of the second 

 electron will be that of the ionised atom. This process could obviously 

 be supposed to be repeated, so that spectra originating in doubly- or 

 multiply-ionised atoms might be considered possible. The theory pre- 

 dicted that such spectra would be characterised by series systems for 

 which the series constant would be 4 R, 9 R, 16 R, and so on, for atoms at 

 successive stages of ionisation. The spectrum of ionised helium, which 

 had previously been obtained without its identification as such,^ had 

 indeed already contributed very materially to the formulation of the new 

 theory. 



Bohr's theory proved a great stimulus to experimental spectroscopy 

 as well as to theoretical investigations. Among the first-fruits was the 

 experimental verification of the predicted 4 R value for the series constant 

 in the spectra of ionised magnesium, calcium and strontium.^ Next, 

 Sommerfeld's well-known extension of the theory of the hydrogen 

 spectrum by taking account of the relativistic variation of the mass of 

 the electron with its orbital velocity predicted a fine structure of the 

 lines of hydrogen and of ionised helium which was almost immediately 

 verified by Paschen's remarkable observations of the structure of ionised 

 helium lines under very high resolving power. 



A general explanation of the existence of several types of series 

 S, P, D . . .in the spectra of more complex atoms immediately followed, 

 namely, that such types of series are to be attributed to the action on the 

 series electron of a perturbing field due to the presence of other electrons 

 in the atom, producing a precessional motion similar to that associated 

 with the relativity effect, but of very much greater value. Two quantum 

 numbers thus became necessary in order to describe the motion of the 

 series electron. They are usually written as n,., where n is the 

 ' principal ' quantum number and k the so-called ' azimuthal ' quantum 

 number. In a simple ellipse, n determines the semi-major axis, and k 

 the semi-minor axis ; kJij2Tz is the angular momentum of the electron. 

 In the case of the simpler spectra first dealt with, the same quantum 

 numbers could be used to specify the characters of the corresponding 

 spectral terms, so that we have k—1, 2, 3 . . . corresponding to the term 

 types S, P, D. . . . 



« A. Fowler, Mon. Not. R.A.S., vol. 73, p. 62 (1912). 



» A. Fowler, Phil. Trans., A, vol. 214, p. 225 (1914). 



The verification of 9 R for doubly-ionised aluminium by Va,schen (Ann. d.Phys., 

 vol. 71, p. 142, 1923), and of 16 R for trebly-ionised silicon by A. Fowler (Roy. Soc. Proc, 

 A, vol. 103, p. 413, 1923), followed in due course. 



