Apeil 6, 1917] 



SCIENCE 



329 



feld^"^ extended Bohr equations in the en- 

 deavor to account for this structure on the 

 basis of elliptieity in some of the orbits, 

 and Paschen^"* by measurements on the 

 structure of the complex helium lines has 

 obtained so extraordinary checks upon this 

 theory that e/m comes out from his meas- 

 ments to within a tenth per cent, of the ac- 

 cepted value. 



A further prediction made by the theory 

 and discovered as soon as looked for was 

 the relation between the lines of two suc- 

 ceeding series of this sort: 



This should hold accurately from the 

 energy relations between the orbits whether 

 there be one or many electrons in the atoms. 

 I have been able to find no case of its fail- 

 ure, though the data upon which it may be 

 tested is now considerable. I have also re- 

 cently pointed out" that it is equivalent to 

 the well-known Rydberg-Sehuster law,^^ 

 which holds quite generally among optical 

 series. Finally, the ionizing potential of 

 hydrogen is given by Bohr's equation as 

 13.54 volts while experiment yields 11.5 

 volts. This discrepancy in no way preju- 

 dices the theory, but rather lends it sup- 

 port, for the computed value is for the 

 hydrogen atom while the observed value re- 

 lates to the hydrogen molecule, which in 

 view of the repulsions of its two negative 

 electrons might be expected to be ionized 

 more easily than the hydrogen atom. Sim- 

 ilarly the computed value for helium which 

 has lost one negative is 52.4 volts, but the 

 helium molecule is found experimentally 

 to be ionized at a much lower potential, 

 viz., 20.5 volts. That Bohr computed this 

 latter value at 27 instead of 20.5 volts is 



15 Annalen der Fhysik., 51, 1, 1916. 

 '^f> Annalen der PhysiTc., Oct., 1916. 



17 TJiys. Bev., May, 1917, presented before 

 American Physical Society, December 1, 1916. 



18 Baly, ' ' Spectroscopy, ' ' p. 488. 



not at all serious, since he had to make very 

 particular assumptions to get this result. 



If then the test of truth in a physical 

 theory is large success both in the predic- 

 tion of new relationships and in correctly 

 and exactly accounting for old ones, the 

 theory of non-radiating orbits is one of the 

 best established truths of modern physics. 

 For the present at least it is truth, and no 

 other theory of atomic structure need be 

 considered until it has shown itself able to 

 approach it in fertility. I know of no com- 

 petitor which is as yet even in sight. I am 

 well aware that the facts of organic chem- 

 istry seem to demand that the valence elec- 

 trons be grouped in certain definite equi- 

 librium positions about the periphery of 

 the atom, and that at first sight this demand 

 appears difficult to reconcile with the theory 

 of electronic orbits. As yet, however, there 

 is no necessary clash. Hydrogen and 

 helium present no difficulties, since the 

 former has but one valency, and the latter 

 none. It is to these atoms alone that the 

 unmodified Bohr theory applies, for it 

 treats only the case of a single negative 

 electron rotating about a positive nucleus. 

 That the K radiations of the heavy elements 

 are so accurately predictable from those of 

 hydrogen indicates indeed that close to the 

 nucleus of these elements there lie electrons 

 to which the Bohr theory fairly accurately 

 applies, but the radiations give us no in- 

 formation about the conditions or behaviors 

 of the external electrons which have to do 

 with the phenomena of valency and we have 

 investigated but little the radiating proper- 

 ties of the atoms which possess but few 

 electrons. A further study of the behavior 

 with respect to X-rays of the elements from 

 lithium, atomic number 3, to sodium, 

 atomic number 11, may be expected to 

 throw new light on this problem. 



It has been objected too that the Bohr 

 theory is not a radiation theory because it 

 gives us no picture of the mechanism of the 



