28 BIOLOGICAL EFFECTS OF RADIATION 



whether one should eschew such a picture and simply imagine the entire 

 process of inf ailing and emergence taking place en bloc, may be left to the 

 individual's taste. 



PRODUCTION OF LIGHT OF THE OPTICAL SPECTRUM : ELECTRICAL 



MEANS 



The theoretically most intelligible way of producing light of the optical 

 spectrum is to bombard a gas with electrons of definite and controllable 

 speed. Suppose such a gas as mercury vapor or sodium vapor, which 

 when cool, rarefied, and unexcited (page 16) displays an absorption 

 spectrimi consisting of a line series. Denote by vi, v^, vs, . . . the 

 frequencies of the successive lines of this series, starting from that of 

 lowest frequency; by vn^ the limit frequency. As I have said already, 

 the quantities hvi, hv^, hvz are the energies of various stationary states of 

 the atom of the vapor in question, referred to that of the normal state as 

 zero of energy. 



If the law of production of spectrum lines were the same in this 

 region as in the X-ray region, an emission spectrum composed of all of 

 these lines should suddenly appear in its completeness when, and not 

 before, the kinetic energy of the impinging electrons attains the value 

 /ij'iini. This, however, is not the case. Under ideal experimental condi- 

 tions these lines appear successively in emission, the first when the energy 

 U of the electrons attains hvi, the second when U attains hv^, and so forth. 

 This is just what is to be expected, in view of the fact that photons having 

 these energies are absorbed by the atoms. The electrons, of which the 

 transitions are responsible for these lines, are able at any time to go from 

 the normal location or "orbit" to any one of the outer orbits. We say 

 that this is because the outer orbits concerned in optical spectra are 

 normally empty, while those concerned in X-ray spectra are normally 

 full. A K electron in a gold atom cannot be transposed into the Lu orbit 

 or shell (these two words are used almost interchangeably) because there 

 is already the prescribed niunber of electrons in this shell and no more 

 can be added. It must either be left altogether alone or thrown clear 

 out of the atom.^i But the electron in the normal state of the hydrogen 



^1 If a photon or an electron of sufficient energy should impinge on an atom just 

 when there happened to be a vacancy in, say, the Lu shell, there is certainly no 

 obvious reason why it should not push a K electron up into that vacancy; but the 

 chance of an impact at just such a moment is so slight that, so far as I know, the 

 effect has not been observed. — It also seems to be possible for K electrons to be 

 transferred into vacant orbits in the periphery of the atom, but the effect is probably 

 of no practical importance (it discloses itself in narrow spectrum lines near an absorp- 

 tion edge). 



