562 



NATURE 



{Oct. 14, 1880 



MULTIPLE SPECTRA^ 

 III. 

 T HAVE endeavoured to show in the previous articles 

 ■*■ that there are many facts which justify the conclusion 

 that the same elementary substance in a state of purity 

 can under different conditions give us spectra different in 

 kind. To those spectra to which svpc'xp^ ,„r.-.- - — "^>'' 



made the "o^- -"■ """ i^K/'frf have been given to 



...aiK. ^heir chief point of difference, which is that in 

 lined spectra we deal with lines distributed irregularly 

 over the spectrum ; while in fluted spectra we deal with 

 rythmical systems. 



This was the first point, and I showed that the idea was 

 suggested that the lined and fluted spectra, though pro- 

 duced by the same substance, were produced by that 

 substance in a different molecular condition. 



I have pointed out that both in lined and fluted spectra 

 taken separately there was evidence of still further com- 

 plication, that is, that a complete lined spectrum of a 

 substance and a complete fluted spectrum of a substance, 

 was the result of the vibration not of one kind of molecule 

 only, but probably of several. 



So that in this view we have to imagine a series, in 

 some cases a long series, of molecular simplifications 

 brought about by the action of heat, and ascribe the 

 spectral changes to these simplifications. 



To understand my contention, and one objection which 

 has been taken to it, in the clearest way, let us suppose 

 that there is a substance which gives us, under different 

 conditions, three spectra, which we will term a, b, and c. 

 My view is that these spectra are produced by three 

 distinct molecular groupings brought about by successive 

 dissociations. On the other hand, it is objected that they 

 are produced by one and the sauic molecule struck, as a 

 bell might be struck, in different ways by the heat waves 

 or the electric current passing among the molecules. 



In my memoir entitled " Discussion of the Working 

 Hypothesis that the so-called Elements are Compound 

 Bodies," I remarked as follows : — 



"I was careful at the very commencement of this paper to 

 point out that the conclusions I have advanced are based 

 upon the analogies furnished by those bodies which, by 

 common consent and beyond cavil and discussion, are 

 compound bodies. Indeed, had I not been careful to 

 urge this point, the remark might have been made that 

 the various changes in the spectra to which I shall draw 

 attention are not the results of successive dissociations, 

 but are effects due to putting the same mass into different 

 kinds of vibration or of producing the vibration in dif- 

 ferent ways. Thus the many high notes, both true and 

 false, which can be produced out of a bell with or without 

 its fundamental one, might have been put forward as 

 analogous with those spectral lines which are produced 

 at different degrees of temperature with or without the 

 line, due to each substance when vibrating visibly with 

 the lowest temperature. To this argument, however, if 

 it it were brought forward, the reply would be that it 

 proves too much. If it demonstrates that the // hydrogen 

 line in the sun is produced by the same molecular group- 

 ing of hydrogen as that which gives us two green lines 

 only when the weakest possible spark is taken in hydro- 

 gen inclosed in a large glass globe, it also proves that 

 calcium is identical with its salts. For we can get the 

 spectrum of any of the salts alone without its common 

 base, calcium, as wc can get the green lines of hydrogen 

 without the red one. 



" I submit, therefore, that the argument founded on the 

 over-notes of a sounding body, such as a bell, cannot be 

 urged by any one who believes in the existence of any 

 compound bodies at all, because there is no spectroscopi'c 

 bieali between acknowledged compounds and the sup- 

 posed elementary bodies. The spectroscopic differences 



* Continued from p. 312. 



between calcium itself at different temperatures is, as I 

 shall show, as great as when we pass from known com- 

 pounds of calcium to calcium itself. There is a perfect 

 continuity of phenomena from one end of the scale of 

 temperature to the other." 



Not only is what may be teiTned the W' '" -•■—"!> 



opposed fo ft.» >— ' -; "'^r> as I endeavoured to 



^.TiJvP m the last paragraphs quoted, but it appears never 

 to have struck the objectors that it is also opposed to 

 the theory of exchanges as it is generally enunciated, 

 on which the whole of our supposed knowledge of 

 extra-terrestrial matter depends. If vapours, when rela- 

 tively cool, do not absorb the same wave-lengths which 

 they give out when relatively hot, what becomes of some 

 of the most noted exploits of our nineteenth-century 

 science ? 



Take the case of sodium. Three distinct spectra have 

 been mapped for it. There is first the yellow line seen 

 in a Bunsen flame, then the green line seen alone in 

 a vacuum tube when the vapour is illuminated by an 

 electric glow, and again there is the fluted absorption 

 spectrurn, without any lines, seen when sodium is gently 

 heated in hydrogen in a glass tube. If we have here 

 the same molecule agitated m different ways, I ask which 

 is the true spectrum of sodium ? And what right have 

 we to say that sodium exists in the sun because the 

 yellow line is represented ? Why do we not rather say 

 that sodium does not exist in the sun because the fluted 

 spectrum is not represented. 



It is not necessary to enlarge upon this point because 

 the difficulty in which the theory of exchanges is thus 

 landed is obvious, while, if we acknowledge diflerent 

 molecular groupings in the vapours of the same chemical 

 substance, and apply the theory of exchanges to each 

 i:;roiiping, then the teachings of that theory become more 

 numerous and important than before. 



It is of course of the highest importance to see 

 whether there is any cxperimentum crucis — any mode of 

 inquiry — by which the theory can be settled one way or 

 the other. 



I submit that the results of experiments based on the 

 following considerations ought to be accepted as throwing 

 light on the question. 



1. At different temperatures the brilliancy of the spectral 

 lines of the same substances as ordinarily observed 

 changes enormously. See if these changes can be pro- 

 duced at the same te///pe/ atnre by employing those experi- 

 mental conditions %vhich will be most likely to bring about 

 different molecular conditions if such exist. 



2. At a low temperature some substances give us few lines 

 while at a high one they give us many. Vapours, there- 

 fore, already glowing with few lines at a low temperature, 

 say in a flame, should give us all their lines when the 

 vapour is suddenly subjected to a high one, say by the 

 passage of a high tension spark. On the bell hypothesis 

 the spectrum should change with the mode of striking. 

 On the dissociation hypothesis this should only happen 

 for the lines of those molecular groupings which are f/om 

 ot/ier considerations held to be more simple. If the flame 

 has brought the substance to its lowest state, the passage 

 of the most powerful spark should not cause the flame 

 spectrum to vary. 



Now what are the " other considerations " above re- 

 ferred to ? This necessitates a slight digression. 



In the /'////. Ti-ans. for 1873' I gave an historical 

 account, showing how, when a light source such as a spark 

 or an electric arc is made to throw its image on the sHt of a 

 spectroscope, the lines had been seen of different lengths> 

 and I also showed by means of photographs how very 

 definite these phenomena were. It was afterwards de- 

 monstrated that for equal temperatures chemical com- 

 bination or mechanical mixture gradually reduced the 



* r/iU. TraTts,^ 1873, p. 254. 



