PROCEEDINGS OF SECTION B. 



99 



D. Inorganic Anhydrides — 



10. Iodic; 11. Arsenious ; 12. Boric. — In no case was the 



rate measurable. That this was due to the rapidity 



of hydration and not to an infinitesimally small velocity 



was proved by the fact that a solution in a sealed cell 



showed no change of conductivity after several days. 



In Table IV. are collected all the constants so far obtained for 



organic anhydrides, together with the dissociation constants of the 



acids. 



Table IV. 



Eoughly, dissociation constant and velocity constant run parallel, 

 and when the former is greater than ' 02, the velocity of hydration of 

 the anhydride is too great to measure at 25 deg. Naphthalic anhydride 

 cannot be measured, although the dissociation constant of its acid is 

 much less than 0^02. The reason for this may be the great insolubility 

 of the anhydride, but still more probable is it that the reason is here 

 the same as that which appears likely to explain the impossibility of 

 measuring the rate of hydration for arsenious and boric oxides, whose 

 acids are very slightly dissociated, viz., the reversibility of the hydra- 

 tion in aqueous solution. What is measured in the method under 

 consideration is really an apparent hydration constant, and this is 

 equal to the sum of the real direct and reverse constants. Hence, 

 although the former may be small, the apparent value may be so high 

 as to correspond to an impossibility of measurement, this being so 

 much more the case the nearer equilibrium h to the dissociated side. 



