28 



SCIENCE. 



"II est bien evident que les cas dont nous venons de 

 parler, ne forment pas une exception a la loi generale 

 enoncee ci-dessus, savoir que chaque corps simple ne peut 

 donner qu'un seul spectre. En effet, si Ton suppose que 

 l'etat allotropique est dii a la constitution moleculaire du 

 corps, soit que les molecules se combinent les unes avec les 

 autres, soit qu'elles s'arrangent entre elles d'une certaine 

 maniere, cet etat allotropique possedera au point de vue 

 spectroscopique, toutes les proprietes significatives d'un 

 corps compose, et par consequent il doit etre decompose de 

 la meme facon que celui-ci par les effets de la decharge dis-. 

 ruptive de l'electricite." 1 



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

 of Pliicker'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 Metal- 

 loids, 3 was driven to the conclusion that many of these 

 bodies must be held to possess two spectra. His conclusions 

 are thus expressed :— 



" Nous avons compare le spectre d'absorption du brome 

 et de l'iode a leur spectre electrique, et cette comparaison 

 nous semble mettre hors de doute la possibility des spectres 

 doubles. . . . 



"Nous avons obtenu, par voie electrique, un spectre 

 primaire de l'iode correspondant a son spectre d'absorption. 

 Le soufre, le selenium et le tellure nous ont offert des spec- 

 tres de combustion tres-analogues aux spectres primaire 

 obtenus par voie electrique, mais differant essentiellement 

 des spectres des lignes. . . . 



" Nous avons produit le spectre primaire de l'azote avec 

 differents corps qui n'ont absolument de commun que 

 l'azote ; nous pensons done avoir demontre qu'il appartient 

 bien reellement a ce metallolde." (Annates de Chemie et 

 de Physique, 4 serie, tome xxviii. pp. 70, 71). 



In 1868 Wullner 4 gave his attention to this subject, and 

 strongly supported Pliicker's view of the existence of 

 double spectra, indicating at the same time that the differ- 

 ence of temperature must be regarded as the sole cause of 

 the phenomenon, adding, however, " a decomposition with 

 further elements is not to be thought of." In the case of 

 hydrogen he showed that the banded spectrum ascribed to 

 acetylene really depended upon a change in the emissive 

 power brought about by an alteration of temperature. 

 Touching oxygen, he showed that three distinct spectra may 

 be obtained, while in nitrogen two are observed. 



I may say that in my early laboratory experiments I was 

 at first led to think that, in the case of metallic vapors, 

 Angstrom's first expressed opinion was correct, and I said 

 so. But after more experience and knowledge had been 

 acquired, I was compelled by the stern logic of facts to 

 abandon it, and I showed, first, that more "orders" of 

 spectra — to use Pliicker's term — were necessary, and then 

 that the line spectrum itself was in all probability com- 

 pound ; that is, that it was in some cases built up by the 

 vibration of dissimilar molecules, some of which might even 

 give us a fluted spectrum, if we could study them alone. 



Although, however, in the views I have expressed on 

 former occasions I have had the advantage of the support of 

 the opinion of Pliicker and Angstrom, and later of Dr. 

 Schuster,'-' not to mention others, I am aware that though 

 there is a general consensus among spectroscopic workers 

 that double spectra cannot be ascribed to impurities, it is not 

 absolute. 



I propose therefore in this place to refer to a special case 

 in which this question has been recently brought prominently 

 forward. 



o 



' Angstrom and Thalcn's " Rccherchcs sur les Spectres des Mctalloides," 

 p, 5. - Nature, vol. xv. p, .07. 



I Ann. dt Chitnit et de PhyMtaut, 1373, vo '- xxviii. p. 1, 



1 /'////. Ma,-., sec. 4, vol. xxxvii. p. 405. 



I have already stated that Angstrom, who was the first to 

 map the line-spectrum of carbon, ascribed the flutings 

 ordinarily seen in the carbon compounds to acetylene. 



Now Attfield, in 1862, as a result of a most carefully con- 

 ducted and admirably-planned set of experiments, came to 

 the conclusion that the flutings were really due to carbon : 

 in short, that carbon, like hydrogen, iodine, sulphur, nitro- 

 gen, and other bodies, had a tinted spectrum as wall as one 

 consisting wholly of lines. 



The work of Attfield will be gathered from the following 

 extract from his paper (Phil. Trans., vol. clii. part 1, p. 221 

 et seq.) : — 



•' On tecently reading Swan's paper by the light that 

 Professors Bunsen and Kirchoff have thrown on the sub- 

 ject, I came to the conclusion that these bands must be 

 due to the incandescent carbon vapor; that, if so, they 

 must be absent from flames in which carbon is absent, and 

 present in flames in which carbon is present ; that they 

 must be observable equally in the flames of the oxide, sul- 

 phide, and nitride as in that of the hydride of carbon ; and, 

 finally, that they must be present whether the incand- 

 escence be produced by the chemical force, as in burning 

 jets of the gases in the open air, or by the electric force, as 

 when hermetically-sealed tubes of the gases are exposed to 

 the discharge of a powerful induction-coil. . . . 



"To establish the absolute identity of the hydro- and 

 nitro-carbon spectra, excluding of course the lines due to 

 nitrogen, they were simultaneously brought into the field of 

 the spectroscope: one occupying the upper, and the other 

 the lower half of the field. 



" This was readily effected after fixing the small prism, 

 usually supplied with spectroscopes, over half of the nar- 

 row slit at the further end of the object-tube of the instru- 

 ment. The light from the oxyhydrocarbon flame was now 

 directed up the axis of the tube by reflection from the little 

 prism, while that from the oxynitrocarbon flame passed 

 directly through the uncovered half of the slit. A glance 

 through the eye-tube was sufficient to show that the char- 

 acteristic lines of the hydrocarbon spectrum were perfectly 

 continued in the nitrocarbon spectrum. A similar ar- 

 rangement of apparatus, in which the hydrocarbon light 

 was replaced by that of pure nitrogen, showed that the re- 

 maining lines of the nitrocarbon spectrum were identical 

 with those of the nitrogen spectrum. In this last experi- 

 ment the sourc^ of the pure nitrogen light was the electric 

 discharge through the rarefied gas. 



" The above experiment certainly seemed to go far to- 

 wards proving the spectrum in question to be that of the 

 element carbon. Nevertheless, the ignition of the gases 

 having been effected in air, it was conceivable that hydro- 

 gen, nitrogen, or oxygen had influenced the phenomena. 

 To eliminate this possible source of error, the experiments 

 were repeated out of contact with air. A thin glass tube 

 1 inch in diameter and 3 inches long, with platinum wires 

 fused into its sides, and its ends prolonged by glass quills, 

 having a capillary bore, was filled with pure dry cyanogen 

 and the greater portion of this gas then removed by a good 

 air-pump. Another tube was similarly prepared with ole- 

 fiant gas. The platinum wires in these tubes were then so 

 connected with each other that the electric discharge from 

 a powe'ful induction-coil could pass through both at the 

 same time. On now observing the spectra of these two 

 lights in the simultaneous manner previously described. 

 the characteristic lines of the hydrocarbon spectrum were 

 found to be rigidly continued in that of the nitrocaibon. 

 Moreover, by the same method of simultaneous observa- 

 tion, the spectrum of each of these electric flames, as they 



1 Dr. Schuster's recently published investigations are as follows;— 

 Air. Lockyer's investigations have shown that most bodies give us a 

 continuous spectrum, as a gas. before they condense, and many at a con- 

 siderable temperature above tne boiling point, Mr. Lockyer has >■ > 

 drawn the conclusion from thtse facts, that tin atomic aggregation of 

 •■, ules is the . attse of tht differtnt orders <>/ .yV, /;-,;. 

 1 ii 1 1 In .1 scontinuous spectra of different orders (Une and hand spectra) 



in du to different molecular combinations. 1 sider to be pretty well 



established, and ogj has led me (and Mr. Lockyer before me) to ex- 

 plain the continuous pectraby the same cause; for the change <>t the 

 continuous spectrum to the line or band-spectrum takes place in exactly 



the same way as the chant;'- of spei u I ol different orders into I Si h "il' 1 i 

 Analogy is nol ■ [Ulde, yet some weight may be javrn toil in a 



1 1 1 like the one under disi ussion, where experimi nl hithi rto lias failed to 

 give .1 decided answer. (Dr. A. Schuster on the Spectra of Metalloids, 



Phil. Trans, Royal Society, 1870. Part i. page 38 and 8y, note). 



