8CHLUNDT — DIELECTRIC CONSTANTS OP PURE SOLVENTS. 383 



one in butyronitrile, notwithstanding the fact that the dielec- 

 tric constant of pyridine is only 61$ of that of butyronitrile. 

 Here, then, we have two further exceptions to the jSTernst-Thom- 

 son rule. But in these eases the exceptions might be explained 

 on the basis of the theory of electrolytic dissociation. Since 

 the molecular conductivity depends upon the speed of the ions 

 as well as the number of ions, i. e., the degree of dissociation, 

 it might be argued that the magnitude of the conductivity at 

 corresponding dilutions, giving as it does the combined effect 

 of these two factors, is therefore not a safe criterion for deter- 

 mining the dissociating power of a solvent, although compari- 

 sons of this kind are frequently made in support of the rule. 

 For example, in the case of the molecular conductivities of sil- 

 ver nitrate in pyridine and butyronitrile, if it be assumed that 

 the speed of the ions in pyridine is materially greater than in 

 butyronitrile, then the molecular conductivity in pyridine 

 may be greater than in butyronitrile, even though the 

 number of dissociated molecules be somewhat less, as is re- 

 quired if we assume the ISTernst-Thomson rule to hold. In 

 the cases cited in Tables VIII and IX the degree of dissoci- 

 ation could not be computed from the electrical conductiv- 

 ity measurements, as no maximum value for the molecular 

 conductivity was obtained. And since, in the case of pyri- 

 dine and benzonitrile solutions, Werner 1 found normal molec- 

 ular weights for silver nitrate by boiling-point determinations, 

 this means of calculating the degree of dissociation could 

 of course not be applied. 2 Hence on the basis of the experi- 

 mental evidence which can be applied in these cases, they must 

 be considered exceptions to the Nernst-Thomson rule. 



In this connection it may be well to note a few other excep- 

 tions to the Nernst-Thomson rule. The cryoscopic and electri- 

 cal conductivity measurements by Zanninovich-Tessarin 3 show 

 that potassium chloride dissolved in formic acid is highly disso- 

 ciated, as one would expect from the high dielectric constant 



1 Zeit. anorg. Chem. 15, 1, (1897). 



2 Compare Kahlenberg, Jour. Phys. Chem. 3, pp. 397-399 on this point. . 

 » Zeit. phys. Chem. 19, 251, (1896) . 



