May 6, 1880] 



NATURE 



fluted bands and the line spectrum (taken as a whole) of 

 a substance really belong to that substance, because if 

 we find that this must be accepted and that it can easily 

 be explained on the view that the two kinds of spectra 

 are produced by different molecular groupings, the fact of 

 other molecular groupings giving rise to a complex line 

 spectrum can be more readily accepted, contrary though 

 it be to modern "chemical philosophy," as taught at all 

 events in the text-books. 



Plucker and Hittorfwere, I beUeve,' the [first to point 

 out that the same chemical substance, when in a state of 

 gas or vapour, gave out different spectra under different 

 conditions. On this point they wrote fifteen years ago : — 



" The first fact which we discovered in operating with 

 our tubes . . . was the following one : — 



" There is a certain iiutnbcr of elementary substances 

 which, when differently heated, furnish two kinds of 

 spectra of quite a differetit character, not having any 

 tine or any band in common. 



"The fact is important, as well with regard to theo- 

 retical conceptions as to practical applications — the more 

 so as the passage from one kind of spectrum to the other is 

 by no means a continuous one, but takes place abruptly. 

 By regulating the temperature you may repeat the two 

 spectra in any succession ad libitum." (Plucker and 

 Hittorf on the Spectra of Ignited Gases and Vapours : 

 Phil. Trans. Royal Society, 1865, part i. p. 6.) 



Angstrom, whose name must ever be mentioned with 

 the highest respect by any worker in spectrum analysis, 

 was distinctly opposed to this view, and in the text which 

 accompanies his Spectre Normal we find the following 

 statement — 



"Dans un Memoire sur les spectres 'doubles' des 

 corps dldmentaires que nous publierons prochainement, 

 M. Thaldn et moi, dans les Actes de la Sociifte des 

 Sciences d'Upsal, nous traiterons d'une manicre suffisam- 

 ment complete les questions importantes qu'on peut se 

 proposer sur cet int^ressant sujet. Pour le present, je me 

 borne ^ dire que les resultats auxquels nous sommes 

 arrives, ne confirment aucunement I'opinion emi:e par 

 Plucker, qu'un corps el^mentaire pourrait donner, suivant 

 sa temperature plus ou moins elev^e, des spectres tout-a- 

 fait differents. C'est le contraire qui est exact. En effet 

 en augmentant successiveraent la temperature, on trouve 

 que les raies varient en intensite d'une maniere tr^s- 

 compliqu(?e, et que, par suite, de nouvelles raies peuvent 

 meme se presenter, si la temperature s'eleve suffisamment. 

 Mais, independamment de toutes ces mutations, le 

 spectre d'un certain corps conservera toujours son 

 caractere individuel." ' 



Angstrom did not object merely on theoretical grounds. 

 He saw, or thought he saw, room to ascribe all these 

 fluted spectra to impurities. 



He was strengthened in this view by observing how, in 

 the case of the spectra of known compounds, there were 

 always flutings in one part of the spectrum or another ; a 

 rapid induction naturally, therefore, ascribed all flutings 

 to compounds. The continuity of the gaseous and liquid 

 states of matter, let alone the continuity of Nature's pro- 

 cesses generally, never entered into the question. For 

 Angstrom, as for the modern chemist, there was no such 

 thing as evolution, no possibility of a close physical rela- 

 tionship between elements, so called, driven to incand- 

 escence from the solid state, and binary compounds of 

 those elements. 



"•Angstrom sur "Le Spectre normal du Soleil," page 39. 



In a memoir, however, which appeared after Angstrom's 

 death, and which, though under a different title, was in 

 all probability the one referred to, this opinion was to a 

 large extent recalled, and in favour of Plucker's view, in 

 the following words : — 



"... Nous ne nions certainement pas qu'un corps 

 simple ne puisse dans certains cas donner differents 

 spectres. Citons, par exemple, le spectre d'absorption 

 d'iode que ne ressemble en aucune fagon au systeme des 

 raies brillantes du meme corps, obtenues au moyen de 

 I'dlectricitd ; et remarquons de plus qu'en gdn(fral tout 

 corps simple, presentant la propriete' d'allotropie, doit 

 donner a I'L-tat d' incandescence des spectres differents, 

 pourvu que la dite propridt<^ de la substance subsiste non 

 seulement a I'e'tat gazeux du corps, mais encore a la tem- 

 p(frature mC-me de r incandescence. . . . 



" Le soufre solid possede, somme on salt, plusieurs 

 etats allotropiques, et, d'apres certaines observations, ce 

 corps, meme a son etat gazeux, prendrait des formes 

 diffi^rentes. Par consequent, en supposant que ccla soit 

 vrai, le soufre gazeux doit donner plusieurs spectres 

 d'absorptio, tandis que la possibility d'un seul on de 

 plusieurs spectres briUants dependra de la circonstance 

 suivante, savoir si les etats allotropiques plus complexes 

 de cette substance supporteront la temperature de 1' incan- 

 descence, avant de se decomposer. 



"Ilestbien evident que les cas dont nous venons de 

 parler, ne forment pas une exception h la loi generale 

 dnoncde ci-dessus, savoir que chaque corps simple ne 

 peut donner qu'un seul spectre. En effet, si Ton sup- 

 pose que I'dtat allotropique est du k la constitution 

 mok'culaire du corps, soit que les molecules se combinent 

 les unes avec les autres, soit qu'elles s'arrangent entre 

 elles d'une certaine maniiire, cet dtat allotropique posspdera 

 au point de vue spectroscopique, toutes les proprietes 

 significatives d'un corps compose, et par consequent il 

 doit ctre decompose de la meme fagon que celui-ci par 

 les effets de la de'charge disruptive de I'electricite."! 



I say that in this paper Angstrom recalled his own in- 

 favour of Plucker's view, because (as it has been remarked 

 by Dr. Schuster -) the word " element" is used in a special 

 sense — because in reality allotropic states are classed as 

 compounds, that particular allotropic state which is to be 

 regarded as truly elemental not being stated, nor any 

 reason given why one should be thus singled out. 



In the letter to which I have just referred Dr. Schuster 

 gives an instance in which in order to show that elementary 

 bodies did not really possess two spectra, a double spec- 

 trum was assigned to an acknowledged compound ; the 

 fluted spectra of hydrogen and carbon which differ from 

 each other as widely as fluted spectra can, being both 

 ascribed to acetylene. 



Salet in his admirable work on the Spectra of the 

 Metalloids,^ was driven to the conclusion that many ol 

 these bodies must be held to possess two spectra. His 

 conclusions are thus expressed : — 



" Nous avons compare le spectre d'absorption dubrome 

 et de I'iode a leur spectre dlectrique, et cette comparaison 

 nous semble mettre hors de doute la possibilite des 

 spectres doubles. . . . 



" Nous avons obtenu, par voie eiectrique, un spectre 

 primaire de I'iode correspondant a son spectre d'absorp- 

 tion. Le soufre, le selenium et le tellure nous ont offert 

 des spectres de combustion trcs-analogues aux spectres 

 primaire obtenus par voie electrique, mais differant essen- 

 tiellement des spectres des lignes. . . . 



^ AngstrCm and Thale'n's " Recherches sur les Spectres des Mtftallotdes," 

 p. 1;. * Nature, vol. xv. p. 447. 



3 Ann. dc Chimie ct de Physique, 1873, vol. xxviii. p. i. 



