IX. On the refractive index of gaseous fluorine

Abstract

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 refractivities 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 showing 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 1’000108, or considerably less than that of hydrogen (1.000139). In 1885 he placed it at 1.6. 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 1.0." More recently Moissan and Dewar, 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.