•714 



It should be noticed that in the interval between experiments 3 

 and 4 a finely divided solid mass had deposited (which verv much 

 impedes the tiltration through cotton wool) ; indeed the experiments 

 following have really been carried out with the two solid phases, 

 anhydride and acid; even after experiment 6 a few long needles of 

 anhydride were still readily visible. 



Afterwards the shaking bottle was provided with acid as well as 

 with anhydride so as to determine the position of point c. Now 

 indeed a composition was attained that remained fairly constant for 

 a longer time (see the first two columns of table II). But it seems 

 peculiar that 1. the value found lies but little above the solubility 

 of the [)hthalic acid and 2. that the value was found to differ in 

 different experimental series. In the dissertation of van de Stadt we 

 also find in the second table on page 49 an extremely smooth maximum 

 as the progress of the solubility in the presence of the two solid phases. 



Perhaps an explanation may be found here owing to a peculiar 

 relation between the xelocities of attainment of the homogeneous 

 and heterogeneous equilibria. As this necessitates the knowledge of 

 the proportion of anhydride to acid in the various solutions this 

 proportion was detei-mined. The chemical method applied by Lumiere 

 and Barbiek ^) in the study of the equilibrium in the homogeneous 

 system acetic-anhydride-water proved impracticable here, but the 

 electric conductivity power previously applied by Voerman") and by 

 Rivett and Sidgavick ^) in the study of the progressive change of the 

 reaction in homogeneous systems, seemed to furnish a better method.") 



In order to render unnecessary the repeated withdrawal of large 

 volumes of liquid a small plunging electrode was constructed with 

 a capacity that just required suitable resistances for liquids used in 

 these experiments. 



We made use of de Haen's phthalic anhydride (m.p. 130°. 6 in 

 VAN Eijk's apparatus'! ; phthalic acid was prepared from that anhydride 

 by complete hydratation ; titre and conductivity j)Ower of the saturated 

 solution appeared to be independent of the quantity of solid phase, 

 hence foreign substances were absent. By numerous determinations 

 with concordant results we found for the solution saturated with 

 phthalic acid : 



1) Bull. Soc. Glieni. de France [3] 33, 783 (1905). 



2) Diss. Gi-oningen 1903, Rec. d. Trav. Chim. d. Pays-Bas 23, 265 (1904). 



3) Journ. Ghem. Soc. 97, 732 and 1677 (1910). 



'^) The method also has been applied by Boëseken and his collaboralors, of. 

 Rec. d. Ti-av. Ghim. d. Pays Bas 1912 and these Proceedings. (Note added in the 

 English translation). 



