142 EEPORT— 1882. 



The likeness of the spectra of calcinm and strontium to the spectrum 

 of magnesium is not so apparent, but Ciamician traces it in taking account 

 only of the low temperature lines of strontium and calcium. The complete 

 spectra of calcium, strontium, and barium are also homologous according 

 to Ciamician — that is to say, we can find in each spectrum groups of lines 

 corresponding to each group in the other two spectra. But the comparison 

 seems less certain, for in spectra having such a large number of lines a 

 little ingenuity will always discover certain likenesses. We do not mean 

 to imply that the resemblances pointed out by Ciamician are imaginary, 

 but only that spectra containing many lines ought not to be taken as tests. 

 The same remarks apply to the remaining groups of elements studied by 

 Ciamician, and we therefore only give his conclusions. 



1. The spectra of oxygen, sulphur, selenium, and tellurium are homo- 

 logous. Corresponding groups are displaced towards the violet with 

 increasing atomic weight. (This last statement seems disproved by 

 Ciamician's own drawing in the case of tellurium, for out of 13 gi'oups 

 of lines, the 10 last ones are displaced towards the red.) 



2. The spectra of phosphorus, arsenic, and antimony are homologous. 



3. The more refrangible parts of the spectra of the nitrogen group are 

 homologous to the more refrangible parts both of the oxygen and of the 

 chlorine group. 



4. The less refrangible parts of the spectra of the oxygen group are 

 homologous to the less refrangible parts of the spectrum of the calcium 

 group. 



The conclusion which Ciamician draws from these facts as to the con- 

 stitution of the various elements and the probability of their ultimate 

 decomposition lie, fortunately, outside the range of the present report. 



We add to the various facts already mentioned the great similarity 

 of the spectra of zinc and cadmium, which has often struck spectro- 

 scopists. 



Professors Liveing and Dewar^ have drawn attention to certain re- 

 lations of wave-lengths which recur in the spectra of lithium, magnesium, 

 and of some lines which are often observed in the spectrum of the chromo- 

 sphere ; they draw from this fact the probable conclusion that these 

 chromospheric lines all belong to the same substance. We mention this 

 fact, as it is one of the first attempts to use the similarity of spectra as a 

 foundation for further conclusions ; but we doubt whether many of those 

 conversant with solar matters will agree with Professors Liveing and 

 Dewar. The way in which these different lines appear on difl'erent 

 occasions seems to suggest very strongly, if not to prove absolutely, that 

 the celebrated green line of the corona belongs to a difl'erent element to 

 that which gives rise to the other chromospheric lines. There is even 

 a diSerent behaviour apparent between the yellow and blue line referred 



blance whatever to the spectrum which Messrs. Dewar and Liveing call the spectrum 

 of hydrocarbons ; and the spectrum which Messrs. Dewar and Liveing call the 

 hydrogen-magnesium spectrum is the spectrum which Ciamician calls the band- 

 spectrum of magnesium ; so that when Ciamician writes that the spectrum of hydrogen- 

 magnesium resembles the spectrum of hydrocarbon, he really makes an altogether 

 different and independent statement when Messrs. Liveing and Dewar make the same 

 remark. But Ciamician quite agrees as to the resemblance pointed out by Messrs. 

 Liveing and Dewar, only he expresses the fact by saying that the spectra of the first 

 order of magnesium and carbon resemble each other. 

 ' Proc. Boy. Soc. xxviii. p. 475 (1879). 



