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inutual attraction-constant a{1 in van nr.it Waals' equation will give a niininmin in the vapourpressures and the phenomeiion may therefore occur witli normal mixtures. The abiiormal vapourdensity iiaturalh suggests the formation of doublé molecules in the vapour and therefore a portion in the liquid (compare the case ot acetic acid), hut again we need a trustworthy criterion to decide between high attraction andassociation. However probable the association in Fhiedel's mixtures may appear, one cannot decide between the two possibilities with the same sharpness as for single substances.