April 6, 1917] 



SCIENCE 



325 



other words, hydrogen's ordinary radia- 

 tions are its X-rays and nothing more. 

 There is also an M series for hydrogen dis- 

 covered by Pasehem in the ultra-red. This 

 in itself makes it probable that there are 

 series for all the elements of longer wave- 

 length than the L series, and that the com- 

 plicated optical series observed with me- 

 tallic ares are parts of these longer wave- 

 length series. As a matter of fact an M 

 series has been found for six of the elements 

 of high atomic weight. Thus the Moseley 

 experiments have gone a long way toward 

 solving the mystery of spectral lines. They 

 reveal to us clearly and quite certainly the 

 whole series of elements from hydrogen to 

 uranium, all producing spectra of remark- 

 able similarity, at least so far as the K and 

 L radiations are concerned, but scattered 

 regularly through the whole frequency re- 

 gion, from the ultra-violet, where the K 

 lines for hydrogen are found, clear up to 

 frequencies (92)^ or 8,464 times as high. 

 There can scarcely be a doubt that this 

 whole field will soon be open to our ex- 

 ploration. How brilliantly, then, have 

 these recent studies justified the predic- 

 tions of the spectroscopists that the key to 

 atomic structure lay in the study of spec- 

 tral lines. The prophets little dreamed, 

 however, that the study of spectral lines 

 meant the study of X-rays. But now, 

 through this study, a subatomic world 

 stands revealed to us in simpler form than 

 one could have imagined. For the atoms 

 are now seen to be, in their inner portions 

 at least, remarkably similar structures, with 

 central nuclei which are exact multiples of 

 the positive electron, surrounded in each 

 case by electronic orbits which have cer- 

 tainly, so far as the inner ones are con- 

 cerned, practically the same relations in all 

 the elements, the radii of all these orbits 

 being inversely proportional to the central 

 charge or atomic number. 



So far nothing has been said about a 



quantum theory or a Bohr atom. The re- 

 sults have followed from the known prop- 

 erties of assumed circular electronic orbits 

 combined with Moseley 's experimental law, 

 as he interpreted it, and supplemented by 

 the single additional assumption that the 

 observed frequencies are proportional to 

 the orbital frequencies. If they suggest, 

 however, that the experimental facts do not 

 necessitate the quantum theory for their 

 complete interpretation, the consideration 

 of the energy relations involved — ^these 

 have been entirely ignored thus far — re- 

 veals at once the futility of that hope, or of 

 that fear, according to the nature of your 

 predilections with regard to the theory of 

 quanta. For the experimental facts and 

 the law of circular electronic orbits have 

 limited the electrons to orbits of particular 

 radii. But the energy principle does not 

 permit them to be so limited without a sud- 

 den or explosive loss of energy whenever 

 the orbit is obliged to change. Suppose, 

 for example, that a cathode ray strikes the 

 atom and knocks out any electron from a 

 particular orbit. When this or some other 

 electron returns from infinity to this orbit, 

 it must in this act adjust its energy to the 

 only value which is consistent with this 

 orbit and its characteristic frequency. 

 Hence in the act of readjustment it must 

 radiate a definite quantity of energy. Or 

 again, suppose that the nucleus loses a beta 

 ray through the radioactive process. 

 Every electronic orbit must then adjiist 

 itself to the new value demanded by Mose- 

 ley 's law. But this it can not do if its 

 energy is conserved. The only way to per- 

 mit it to do so is to let it radiate a definite 

 amount of energy in the act of adjustment. 

 This suggests that each emission of a beta 

 ray by a radioactive substance must be ac- 

 companied by a whole series of characteris- 

 tic gamma rays corresponding to each 

 changed orbit. The emission of an alpha 

 particle, on the other hand, would require 



