Decembee 29, 1916] 



SCIENCE 



909 



drops to zero, and producing multiply 

 charged ions in the beginning and singly- 

 charged ions toward the end of their 

 course, would apparently account for all 

 the observed effects. 



While the experimental evidence seems 

 to favor the idea that lines are emitted by 

 positively charged centers, there is no a 

 priori reason why neutral or even negative 

 ions should not emit line spectra. It is 

 quite possible that the canal ray lines 

 which Stark attributes to singly charged 

 ions may be emitted at the instant of neu- 

 tralization ; but we can not escape the con- 

 clusion that spark lines at least are emitted 

 by positive ions unless we accept the im- 

 probable view that a multiple charge may 

 be instantaneously entirely neutralized. 

 Lenard inferred from the distribution of 

 emission centers in the arc that the lines 

 of the principal series are emitted by neu- 

 tral atoms, those of subordinate series and 

 spark lines by multiply charged atoms. 

 Wien and others have suggested that line 

 spectra may be emitted by molecules, but 

 this seems improbable. On the other hand, 

 we must admit the possibility of negatively 

 charged centers which would probably ex- 

 ist only under exceptional conditions. 

 Nicholson has, with success, assumed the 

 existence of positive, neutral, and negative 

 centers in accounting for the spectrum of 

 the corona. 



The fundamental importance of reach- 

 ing definite conclusions as to the magnitude 

 of the electric charge of emission centers is 

 evident when we remember that any theory 

 must take this into account. Bohr's theory 

 rests upon the assumption that series lines 

 are emitted by electrons previously de- 

 tached as they return to equilibrium posi- 

 tions determined by the resultant charge 

 of the system. In the case of hydrogen, if 

 there be but one detachable electron, the 

 radiating system must be neutral. If it 

 can be shown without question that the 



emission centers of the Balmer series are 

 positively charged, some modification of 

 the theory seems necessary. Furthermore, 

 if the centers are thus deprived of the one 

 detachable electron, we must accept Stark's 

 view that the series emission is due to elec- 

 trons which can not be detached. Fulcher 

 has pointed out the necessity for a similar 

 conclusion with respect to helium. Some 

 of its lines are attributed to doubly 

 charged atoms ; but these are identical with 

 alpha particles, the nuclei of the atoms, 

 from which the radiation must be emitted. 



Beyond the probable fact that band spec- 

 tra are usually emitted by neutral systems, 

 there is little evidence upon which we may 

 rest a theory. Emission may accompany 

 the neutralization of a positively charged 

 molecule by an electron or may be the re- 

 sult of internal vibrations due to colli- 

 sions, without complete ionization. Stark 

 believes that the band emission is due to 

 the detachable valency electrons, although 

 the coupling between them and more firmly 

 bound electrons may cause the latter to 

 take part. 



Evidence supporting Stark's views is to 

 be found in absorption spectra. Hydrogen 

 shows no absorption until it is ionized by 

 a current. The cold vapors of the alkali 

 metals and of mercury show line absorp- 

 tion, but their susceptibility to the photo- 

 electric effect indicates how ionization may 

 be the prelude to absorption. All the cor- 

 responding emission lines appear to be due 

 to singly charged emission centers. Ab- 

 sorption of the lines due to multiple 

 charges does not take place until the vapor 

 is highly ionized by electric discharges or 

 high temperature. Substances which show 

 band absorption under ordinary conditions, 

 such as iodine, do not appear to be ionized 

 when either emitting or absorbing. Both 

 processes appear to be due to neutral sys- 

 tems. In such cases emission must be due 

 to internal disturbances, without ioniza- 



