TUAN.SACTIUXS OF SKCTION II. 



66; 



m our 

 iiljlc (Min- 

 I'in 1h;0, 

 ;oiiceniiiig 

 tli(> dis- 

 a moment 

 ; us the 

 oilers tln' 

 toniic and 



reseavdips 

 direction 

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 observed 

 itter only 

 res necos- 

 spectni — 

 metals— 

 on in the 

 ht of the 

 spectra of 



(lilTerent elomonts is far from Ijoiiifr placed on a satisfactory ha.-is, and in spite of 

 (he resenrclies of J^ecoq do Boisbandran, Ditto, Troost anil Ilautefeiiille, Ciamiciaii 

 and others, it. cannot be said that us yet definite proof lias been given in support 

 of the theory that a causal coiniection is to be found between the eniis.siuii 

 spectra of Ihe several elements belonjring to allied jrroups and their atomic weijrlits 

 or other chemical or ))hysical properties. ]n certain ol' the single elements, liow- 

 I'ver, the connection between the spectra and the nvilecular constitution can be 

 traced. lu the case of sulphur, for example, three ilLstinct spectra are known. 'I'ho 

 iirst of these, a continuous one, is exhibited at tem])eratures below .jOU', when, 

 as wo know from JJunuis' experiments, tin; density of the vapour is three times 

 t!'0 nornnil, sbowiug that at this temperature the molecule consists of six atoms. 

 Tlie second spectrum is .s(;en wlien the lemjierature is raised to above 1,000 , when, 

 an Deville and Troost have shown, the vapour reaches its nornuil density, and 

 the molecule of sulphur, as with most other gases, contains two atoms, and this 

 is a band spectrum, or one characterised by chaimelled sjiaces. Together with this 

 l)and spectrum, and esjiecially rounil the negati\e pole, ii spectrum of brigbt lines 

 is observed. This latter is doubtless due to tlie vibrations of (he single atoms of the 

 dissociated molecule, the existence of traces of a ban<l spectrum demonstrating the 

 fact that in some ]iarts of the ilischarge the tension of dissociation is insullicient to 

 prevent the reunion of the atoms to form the molecule 



To this instance of the liglit tlirown on molecular relations by changes in thn 

 spectra, others may be added. Thus the low-teniperatni'e sjiectrnm of channelled 

 spaces, mapped by Schuster and myself, in the cast- of potassium, corresponds to 

 the molecide of two atoms and to the vapour-density of .sevimty-nine, as observed 

 by Dewar and Ditlmar. Again, both oxygen and nitrogen exhibit two, if not 

 three, distinct spectra : of these the line spectrum seen at the highest temperatures 

 corresponds to the atom; the band spectrum seen at intermediate temperatures 

 represents the molecule of two atoms ; whilst that observed at a still lower point 

 would, as in the case of sulphur, indicate the existence of a more complicated 

 molecule, k'nown to us in one instance as ozone. 



That this explanation of the cause of these dilli.'rent spectra of an element is 

 the true one, can be verified in a remarkalile way. Contrary to liie general rule 

 ;!inoug-st those elements which can readily be volatilised, ami with which, therefore, 

 low-temperature sj)ectra can be studied, mercury exhibits but one spectrum, and 

 that one of bright lines, oi*, acct>rding to the jireceding theory, a spectrum of atoms. 

 80 that, judging from spectroscopic evidence, we ini'er that the atoms of mercury 

 do not unite to form a molecule, and we should predict that the vapour-density of 

 mercury is only half its atomic weight. Such we know, from chenucal evidence, is 

 really the case, the molecule of mercui'v being iilentical in weight with i!s atom. 



The cases of cadmium and iodine require further elucidation. The molecule of 

 gaseous cadmium, like that of mercury, consists of one atom ; probably, tlieridbre, 

 ilio cadmium spectrum is also distinguished by one set of lines. Again, the 

 niolecnle of iodine at i,!200" separates, as we know from Victor Meyer's researches, 

 into single atoms. Here spectrum unalysis may come again to our aid ; but as 

 Schuster remarks, in his report on the spectra of the non-metallic elemc-nts, a more 

 extensive series of experiments than those already made by Ciamician is required 

 Ijet'oro any definite opinion as to the connection of the different iodine spectra with 

 the molecular condition of the gas can be expressed. 



It is not to be Avondered at that these relations are only exbibited in the case 

 of a few elements. For most of the metals the vapour-density remains, and pro- 

 hably will remain, an unknown quantity, and therefore the connection between any 

 observed changes in the spectra and the molecular weights must also remain un- 

 known. The remarkable changes Avhich the emission spectrum of a single eh-iuent — 

 iron, for instance — exhibits have been the subject of much discussion, experimental 

 and otherwise. Of these, the phenomenon of long and short lines is ona of the 

 most striking, and the explanation that the long lines are those of low temperature 

 appears to meet the facts satisfactorily, although the effect of dilution, that is, a 

 reduction of the quantity of material undergoing volatilisation, is, remarkably 

 enough, tbe same aa that of diminution of temperature. Thus it is possible, by tlie 



I 'r 



