. Stœchiométrie . );Dolezalek, Za^ physik. Chem., 64, 727 (1908); 71, 191 (1909); 83, 40 (1913); Campbell, Trans. Faraday SOC, II, 91 (1915); Tinker, Phil. MAG., 32, 295(1916) ; Worley, Trans. Chimique SOC, 105, 273 (1914); Kremann, Monatsh., 37,369 (1916); Hildebrand, Journ. Amer. Chimique SOC, 38, 1452 (1916). Les MÉLANGES 257 à avoir des valeurs finies, et la formule simple p = p^u et/b ^/BCI - m) n'ont pas été trouvés pour être applicables; les relations entre la composition moléculaire et la pression, qu'elles soient totales ou partielles, sont, en fait, représentées par des courbes qui pourraient être bien réprimées par les formules données

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. Stoichiometry . );Dolezalek, Za^ physik. Chem., 64, 727 (1908); 71, 191 (1909); 83, 40 (1913);Campbell, Trans. Faraday Soc, II, 91 (1915); Tinker, Phil. Mag., 32, 295(1916); Worley, Trans. Chem. Soc, 105, 273 (1914); Kremann, Monatsh., 37, 369 (1916); Hildebrand, Journ. Amer. Chem. Soc, 38, 1452 (1916). MIXTURES 257 to have finite values, and the simple formulse p = p^u and/b ^/bCi - m) were not found to be applicable; the relations be-tween molecular composition and pressure, whether total or partial, are, in fact, represented by curves which could be well representedby the formulae given on p. 256. In the case of associating sub-stances, Zawidskis formulae could only be applied if the averagemolecular weights of the associating substances under the varyingconditions of the experiment were known. Investigations of the relation between the composition of liquidand vapour have, as a rule, been carried out at constant temperature, but in practice a liquid is almost always distilled under constant 200 mm.. O 20 40 60 80 100 Molecules per centof propylene dibromide Fig. 65. pressure. The ratio -^ varies slightly with the temperature even for closely related substances, so that if Browns formula were strictlytrue for a distillation at constant temperature, it could not be appli-cable without slight error to an ordinary distillation under constant pressure, or rather the value of -^ ought, theoretically, to be calcu- lated for any given temperature during the distillation. Distillation of Mixtures.—It has been stated that for closely related liquids. Browns formula, —^ = ^ x -13 in all probability Mb Mb holds good with a close approximation to accuracy, and that the con- 17 258 SrOICHIOMETR Y stant c is probably equal to the ratio of the vapour pressures -^ at the boiling point of the mixture. The value of the constant is alwayssuch that the vapour is richer than the liquid in the more volatilecomponent. The distillate obtained by condensation of the vapourwill theref

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