The molecular weights of a number of metals have been determined by Raoult's vapour-pressure method, vie., by ascertaining the depression of the vapour-pressure of the solveiit produced by a known weight of dissolved substance. The connexion between osmotic pressure and depression of vapour-pressure has been shown by van't Hoff, and in a recent number of the Zsitschrift fi;r physiknlische Chemie by Arrhenius ; the method of the latter is characterised by great simplicity. It has also been

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... atically developed by Planck (see also Zeitschr. f. phys. Chem., 1888? 405). The relation between molecular weight and depression is expressed by the equation where IT is the molecular weight to be found; W' the molecular weight of tlie solvent; P/100 the percentage weight of the dissolved substance in solution ; p the vapour-pressure of the solution : and d the depression in the vnpour-pressure of the solvent produced by adding the substance dissolved. It will be noticed that the relation is independent of temperature. The svlvent employed was liquid mercury ; the temperatures 260" and 270" for a few substances, and the boiling point of mercury for b