THE investigation of the refi-actiou equivalents of liquids by Gladstone and others, has led to results which have so materially advanced our knowledge of moleciilar s tructuro that it is perhaps remarkable t h a t no similar results have yet been obtained with crystalline subs tames. During the last 30 years, laborious determinations of the densities and refractive indices of long series of salts have been made by Topsije and Christiansen, Soret), Tutton, and others, often with the express

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... ct of obtaining a direct relationship between refracttire power and chemical coniposition ; this, however, the most important aim of such work haa not hitherto been attained. Proof is advanced i n the present paper that the molecular refractions of crystalliue singly or doubly refracting salts are practically additive quantities, so that the inter-relationship of the refraction constants arid the density, on the one hand, and the composition of a salt, on the other, is of an extremely simple nature, and that on this assumption the refraction constants of a large number OE salts examined by the authors named above can be very closely correlated. The refractive index r , either of aliquid or of an isotropic solid, does not vary with the direclion, and may be a t once combined with the molecular volume in any of the usual ways in order to obtain the molecular refraction. Consequently, using the Gladstone formula,