1^6 



NATURE 



[April 12, 191 7 



of subardinate 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 centres 

 which would probably exist only under exceptional 

 conditions. Nicholson has, with success, assumed the 

 existence of positive, neutral, and negative centres in 

 accounting for the spectrum of the corona. 



The fundamental importance of reaching- definite 

 conclusions as to the magnitude of the electric charge 

 of emission centres 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 detached as they 

 return to equal ihriuin positions 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 centres of the 

 Balmer series are positively charged, some modifica- 

 tion, of the theory seems necessar}-. Furthermore, if 

 the centres are thus deprived of the one detachable 

 electron, we must accept Stark's view that the series 

 emission is due to electrons which cannot 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 spectra are 

 usually emitted by neutral systems, there is little 

 evidence upon which we may rest a theory. Emis- 

 sion may accompany the neutralisation of a positively 

 charged molecule by an electron or may be the result 

 of internal vibrations due to xollisions, without com- 

 plete ioniisation. Stark believes that the band emis- 

 sion 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 absorp- 

 tion unti.i it is ionised by a current. The cold vapours 

 of the alkali metals and of mercury show line absorp- 

 tion, but their susceptibility to the photoelectric effect 

 indicates how ionisation may be the prelude to ab- 

 sorption. All the corresponding emission, lines appear 

 to be due to singly charged emission centres. Absorp- 

 tion of the lines due to multiple charges does not 

 take place until the vapour is higEly ionised hN electric 

 discharges or high temperature. Substances which 

 show band absorption under ordinary conditions, such 

 as iodine, do not appear to be ionised when either 

 emitting or absorbing. Both processes appear to be 

 due to neutral systems. In such cases emission must 

 be due to internal disturbances, without ionisation. 

 The bands of some substances, such as nitrogen, are 

 not found in absorption under any conditions, and 

 the conditions of their occurrence Indicate that the 

 emission bands are due to the recombination of a 

 detached electron with a .positive molecule. The nega- 

 tive pole bands appear under the same conditions as 

 spark lines, and it seems not Improbable that they 

 are due to the neutrallsabion of a doubly charged 

 molecule. 



The spectral differences attending different stages 

 of ionisation are well illustrated bv some recent ex- 

 periments. Franck and Hertz found that mercury 

 vapour is Ionised bv a field of 4-9 volts, and then 

 emits the one ultra-violet line 2537. The Einstein 

 relation Ye = 'hv is fulfilled. McLennan and Hender- 

 son, verified this conclusion, and also found that with 

 a field of about 12 volts a second stage of ioniisation 

 occurs, attended by the emission of the many-lined 



NO. 2476, VOL. 99] 



spectrum attributed by Stark to multiple charges. 

 McLennan finds that zinc, cadmium, and magnesmm 

 also give single line spectra which probably conform 

 with Kinstein's equation, which we should not expect 

 to apply in a simple form to the many-line sfKJctrum. 



It appears from such experiments that there is a 

 threshold value of klnestlc energy which must be 

 imparted to an emission, centre before it can radiate, 

 which represents the work of ionisation and is equal 

 to a light quantum. Franck holds that this energy 

 may be devoted either to ionisation or to einission, 

 but that both cannot simultaneously occur. Stark 

 believes that the two are coincident, the emission 

 accompanying the rearrangement of electrons in the 

 atom after one has been ejected. This suggests an 

 explanation of quantum emission involving no depar- 

 ture from accepted electromagnetic theory. 



The spectra of hydrogen and of helium are of par- 

 ticular interest because their atoms are of the 

 simplest type and because it is possible that they are 

 the basic units of which all elements are composed. 

 The Pickering series in stellar spectra was attributed 

 to hydrogen because of its Uiumerlcal relationships 

 with the Balmer series. The study of series rela- 

 tions led Rydberg to predict the occurrence of a 

 principal series for hydrogen beginning at wave- 

 length 4686, and this line was subsequently found in 

 nebular and stellar spectra. After many attempts to 

 reproduce these spectra In the laboratory. Fowler 

 succeeded in 1812, by passing a powerful disruptive 

 discharge (through a mixture of hydrogen and helium. 

 Produced only under such conditions, these must be 

 classed as spark lines; and if Stark's views are 

 correct and if they are really due to hydrogen, that 

 element must have more than one detachable electron. 



In applying his theory to the helium spectrum, and 

 assuming one electron returning to a helium atom 

 from which two electrons have been detached, Bohr 

 obtained a formula which gives lines correspondin.g 

 in position to those of the Pickering and Rydberg 

 series, and also another series almost coincident with 

 the Balmer hydrogen series. This remarlcable con- 

 clusion was strengthened by Stark's discovery of 4686 

 in a helium tube which gave no lines of the ordinan" 

 hydrogen spectrum. He concluded from the canal- 

 ray displacements that the emission centres were 

 doubly charged. Evans also found the first members 

 of all the series assigned to helium by Bohr, in- 

 cluding that corresponding to the Balmer series. In 

 a tube containing no hydrogen. The experimental 

 evidence thus favours Bohr's theory, but we must 

 remember the remarkable way In which the presence 

 of one element may intensify or suppress the spec- 

 trum of another. For example, Lyman found that 

 the ultra-violet series attributed without question to 

 hydrogen is greatly Intensified by the presence of 

 helium. It may be added that Merton hias con- 

 cluded, from a study of the width of 4686, that it Is 

 due to an atom smaller than that of helium. 



Some light may be thrown on this problem by 

 observations such as those made bv Wright and others 

 on the distribution of materials In nebulae, as indi- 

 cated by the lenarth of the nebular lines. Wright finds 

 that usually 4686 Is confined to the nucleus ; helium 

 lines extend further, and those of nebulum and hydro- 

 gen still further. These results favour the view 

 that the elements distribute themselves according to 

 their atomic weights, and that 4686 Is due to an atom 

 at least as heavy' as that of helium. But this Is not 

 'conclusive, because a high temperature line of 

 hydrogen might be found only in the hot nucleus. If 

 we grant the possibility pf a higher degree of 

 ionisation for hydrogen. 



Fundamental questions which are of Importance to 



