909 



groups, favourably situated in regard to each other; this appears 

 directly if the acid H is substituted by metiiyl, but may be indirectly 

 deduced when the negative influence of the medium (boric acid) 

 on the conductivity of the free acid is taken into account. 



Dilution. 



Found. 



After 

 correction 



After having acquired tiiese data the influence on the conductivity 

 of boric acid — » tannic acid was measured. 



To this end tannin of the gall-nut was used (as the only tantiin 

 available at the present moment) which we prepared according to 

 E. Fischer and Freudenberg (loc. cit.) and from which free gallic 

 acid was eliminated as well as possible. 



Supposing the molecular weight to be 1700, the following figures 

 could be deduced from the measurements. (Conductivity of a 0,5 mol. 

 H.Bo, = 27 X 10-6 K.H. units). 



Dilution. 



Innate con- Conductivity 

 ductivity i + . 5 m H3BO3. 



"^Tannin H3BO3 "('^Tannin + '^HaBOa) 



213 



426 



852 



1704 



91.6 

 57.1 

 35.5 

 21.8 



350 

 229 

 153 

 105 



-f 230.3 



143.8 



89.4 



55.1 



Fi'om these tigures we see that the increase of the conductivity 

 is rerij considerable; in a dilution 213 even it is markedly higher 

 than that of the gallic acid methyl ester in the dilution 32. So the 

 molecule of the gallic tannic acid foims a complex boric acid compound, 

 many times stronger than that of the gallic acid methyl ester, which 

 already heightens the acidity so strongly. 



This result in the first place entirely agrees with the structure of 

 this tannin tixed by E. Fischer, there are iO puirs of favourably 

 situated hydroxyl groups in evei'y molecule. 



59 



Proceedings Royal Acad. Amsterdam, Vol. XXI. 



