January 15, 1915] 



SCIENCE 



77 



solar corona — very simple types of atom 

 exist, much more simple doubtless and more 

 amenable to calculation than are the atoms 

 of most terrestrial substances. "While the 

 correspondence between his calculated 

 spectra and those observed at Lick Observa- 

 tory is not so close as is that between theory 

 and observed spectra in the recent work 

 of Bohr, it is important to observe that 

 most of these results are obtained by means 

 of established mechanical principles and 

 without the use of such questionable as- 

 sumptions as the brilliant young Dane 

 cheerfully and confidently makes. 



And now let us consider briefly the work 

 of Bohr. This is set forth in four papers'" 

 published in the Philosophical Magazine 

 between July of last year and March of the 

 present year. He starts with the Euther- 

 ford atom, i. e., a minute positive nucleus 

 with its system of electrons revolving about 

 it, the mass of the atom resident chiefly in 

 the nucleus and the number of electrons 

 approximately equal to half the atomic 

 weight. He admits the difficulty of secur- 

 ing stability in such an atom (as compared, 

 for instance, with Thomson's 1904 atom), 

 but thinks that this diiSculty can be re- 

 moved if we admit the insufficiency of the 

 classical dynamics to explain phenomena 

 involving atomic distances, and introduce 

 Planck's quantum into the equations. He 

 claims that this furnishes a basis not only 

 for a theory of atomic constitution but for 

 that of molecules as well. He differs from 

 Nicholson radically in assuming that when 

 in a state of uniform rotation, the electrons 

 do not radiate. This is not in accordance 

 with our ordinary electrodynamics. Each 

 atom, according to Bohr, has a number of 

 "steady states" during which the electrons 

 revolve uniformly and there is no radia- 

 tion. But in passing from one steady state 

 to another an electron winds inward to^v;^ 

 the nucleus with its frequency increasing. 



Its acceleration meanwhile causes radia- 

 tion, until the electrons settle into another 

 steady state and ceases for the time to. 

 radiate. In its stable state the angular 

 momentum of every electron is the same. 

 This agrees with Planck's idea of discon- 

 tinuous radiation and the amount radiated 

 in one emission for a vibrator of frequency 

 V is Thv where t is some integer and h is 

 Planck's "universal constant." Bohr 

 finds the equation for the relation between 

 the frequency, mass of an electron, charge 

 of electron, t and h. When t is made 2 ia 

 the equation, Balmer's series for hydrogea 

 is obtained, and for t = 3 the infra-redl 

 series which Ritz anticipated and Paseheni 

 found. T = 1 gives a series of lines in the- 

 ultra-violet and t = 4 and 5 in the infra- 

 red, neither of which has yet been observed'. 

 The lines observed by Fowler and by Pick- 

 ering he connects with helium instead of 

 with hydrogen. 



From this equation he also calculates Ryd- 

 berg's number ^° and obtains 3.26 X 10^^. 

 Its observed value is 3.29 X 10^^, so that 

 the agreement of theory with observation is 

 satisfactory. The theory further requires 

 that very low gas density be required for 

 numerous spectrum lines and very great 

 gas volume for sufficient intensity. This 

 probably accounts for the fact that 33 lines 

 of the Balmer series for hydrogen can be 

 seen in celestial spectra while only 12 ap- 

 pear in terrestrial (vacuum-tube) spectra. 



Prom the work of Barkla, and of Geiger 

 and Marsden on the scattering of radiation 

 Bohr accepts the view of van der Broek 

 that the number of electrons in an atom in 

 the neutral state indicates the position of 

 the element in the periodic table. Thus he 

 gives hydrogen one electron, helium two, 

 lithium three, beryllium four, etc. The 

 same number expresses the magnitude of 

 the positive charge on the nucleus. 



It is difficult to pass upon the validity of 



