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XVIII. On the Specific Refraction and Dispersion of Light by 

 the Alums. By J. H.Gladstone, Ph.D., F.R.S* 



IN the Comptes Rendvs of November 17, 1884, there appears 

 a paper by M. Charles Soret, in which he gives the 

 refractive indices of a large number of crystallized alums. 

 They were determined by the method of total reflection, 

 and are measured for the lines of the solar spectrum a, B, 

 C, D, E, by F, and G. As at the same time M. Soret gives 

 the specific gravity of the specimens examined, it is easy to 

 calculate out the specific refraction and dispersion of these 

 crystals, and it occurred to me that the observations might 

 serve three purposes. They might test the truth of certain 

 physical laws ; they might arbitrate between myself and 

 KanonnikofF as to the refraction-equivalent of certain metals ; 

 and they would give values for indium and gallium, which 

 have not hitherto been obtained. Through the kindness of 

 M. Soret I have received his full paperf , with some further 

 information, and observations on three additional alums not 

 yet published, viz. the Rubidium Gallium, Rubidium Indium, 

 and Caesium Chromium alums. 



In order to bring M. Soret's figures into comparison with 

 my own, which are always based on the line A of the spec- 

 trum, it was necessary to reduce his figures accordingly. This 

 was easily done, as the line A falls short of a by almost exactly 

 the same amount as that one falls short of B. The following 

 table gives the specific gravity of the selected crystals ; the 

 refractive indices (fi) for A calculated to the fourth place of 

 decimals, and also those for G ; the specific refraction for A, 



i. e. the index —1 divided by the density, \~^ — ); the spe- 

 cific dispersion A to G, i. e. the difference between the specific 

 refraction of these two lines, ( ^ A ) ; and the refraction- 



equivalent for A, i. e. the specific refraction multiplied by the 

 atomic weight, ( P , ). The general formula of the alums 



is R 2 , Al 2 , 4 S0 4 , 24 H 2 ; the aluminium being replaceable 

 by indium, gallium, chromium, and iron. 



* Communicated by the Physical Society : read June 27, 1885. 



t u Recherches sur la Refraction et la Dispersion des Aluns crystallises/' 

 Archives des Sciences Physiques et Naturelles y vol. xii. p. 553, and vol. xiii. 

 p. 5. 



