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XLT. On Relations between the Lines of various Spectra. 

 By W. N. Hartley, F.R.S., Professor of Chemistry, 

 Royal College of Science, Dublin. 



IN a paper published in the Philosophical Magazine for 

 July 1890 (vol. xxx. p. 33), by Mr. J. S. Ames, " On 

 Relations between the Lines of various Spectra, with special 

 Reference to those of Cadmium and Zinc, and a Redetermi- 

 nation of their Wave -lengths," the author refers to two of my 

 papers, the titles of which follow :■— " Notes on certain Photo- 

 graphs of the Ultra-violet Spectra of Elementary Bodies,"" J. 

 Chem. Soc. vol. xli. p. 84, 1882 ; and " On Homologous 

 Spectra," loc. cit. vol. xliii. p. 390, 1883. The substance of 

 these papers may be stated in a few words. 



I observed a renr.irkable likeness between the individual 

 lines in the spectra of different elements belonging to the 

 same group, but striking differences between the characters 

 of the lines belonging to elements of different groups. The 

 spectra of the alkali metals were quite different from those of 

 the magnesium group. These latter differed from the iron 

 group. The aluminium, indium, thallium spectra differed 

 from any of the others. The metalloids tellurium, arsenic, 

 and antimony had distinct characters in common. The dif- 

 ferences in the lines of various spectra observed were, first 

 in intensity of chemical action, secondly in length, thirdly 

 in extension above and below the points of the electrodes, and 

 fourthly in breadth or shape, as, for instance, with or without 

 a nimbus. These physical characters were first described by 

 me in the Scientific Transactions of the Royal Dublin Society, 

 vol. i. p. 231, 1881. So important were they considered, 

 that a large amount of labour was incurred in giving a minute 

 description to each line in the tables of wave-length measure- 

 ments made by Mr. Adeney and myself (Phil. Trans, vol. 

 clxxv. p. 63, 1884). Only two other observers have given 

 particular descriptions of the physical characters of lines in 

 spectra, namely, Messrs. Thalen and Lecoq de Boisbaudran. 

 A community of characteristics in spectra was recognized 

 as being due to a similarity in the properties of elements be- 

 longing to the same group when classified according to the 

 Periodic Law. Elements with properties in common likewise 

 exhibit spectra with similar groupings of lines ; but the dis- 

 persion of the lines and the refrangibility of the strong lines in 

 each group vary with the atomic mass of the elements. The 



* Communicated bv the Author. 



