MIXTURES OF H YDROOIILORIC AC'ID AND METHYLETHER. 311 



niutual attraction -constant rt,^ in van df.r Waals' eqnation will give 

 a niininuun in thc vapourpvessures.and tlie phenomenon inay therefore 

 occur with normal mixtures. Thc abnormal vapourdcnsity naturally 

 suggests the formation of double molécules in tlie vopour and therefore 

 a portion in the liquid (compare the case of acetic acid), but again \ve 

 need a trustwortliy criterion to décide between high attraction and asso- 

 ciation. However probable the association in -Friedel''s mixtures may 

 ap})ear^ one cannot décide betweeu the two possibilities with the same 

 sharpness as for single substances. 



