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IX. On the Refractive Index of Gaseous Fluorine. 



By C. CUTHBERTSON and E. B. R. PRIDEAUX, M.A.. JB.Sc. 



Communicated by Sir WILLIAM RAMSAY, K.C.B., F.K.S. 



Received June 5, Read June 8, 1905. 



THOUGH fluorine was isolated by M. MOISSAN as long ago as 1886, no attempt has 

 hitherto been recorded, so far as we are aware, to measure its refractive index in the 

 gaseous state. This omission is the more to be regretted since great interest attaches 

 to the determination. Not only is fluorine the first member of an important group of 

 elements, but its power to retard light, calculated from the refract ivities of its 

 compounds, appears to vary within unusually wide limits, so that the estimates of its 

 refraction equivalent are singularly discordant, and agree only in shmving that it 

 must be remarkably low. 



Thus, Dr. J. H. GLADSTONE* originally gave the refraction equivalents of fluorine 

 and chlorine as 1'4 and 9 "9 respectively, figures which correspond to a refractive 

 index for fluorine of TOOOIOS, or considerably less than that of hydrogen (1 '(100139). 

 In 1885 1 he placed it at I'G. In 1886 G. GLADSTONE^ put down the refraction 

 equivalent at between 0'3 and 0'8, and in 1891 the same observer, with Dr. J. H. 

 GLADSTONE, estimated it as "extremely small, in fact, less than TO.'' More recently 

 MOISSAN and DE\VAR,|| judging from the appearance of liquid fluorine, recorded their 

 belief that the index would be found to be higher than had previously been supposed, 

 though still low in relation to its atomic weight. 



In these circumstances it seemed desirable to attempt to measure the index of the 

 element in the gaseous state, and with this object Mr. CUTHBERTSON visited Paris in 

 January, 1904, and, by the kindness of M. MOISSAN, was enabled to observe the index 

 of a current of fluorine passing through a small hollow prism of copper, the apertures 

 of which were covered by plates of fluor spar. A summary of this work has already 



* 'Phil. Trans.,' vol. 160, p. 26, 1870. 

 t 'American Journal of Science ' [3], XXIX., p. 57, 188-j. 

 J 'Phil. Mag.' [5], XX., p. 483, 1885. 



J. H. GLADSTONE and G. GLADSTONE. 'Phil. Mag.' [5], XXXI., p. 9, 1891. 

 || MOISSAN and DEWAR, 'Proc. Chem. Soc.,' XXXI., p. 175, 1897. 

 VOL. CCV. A 395. 19.10.05 



