Junk 6, 1902.] 



SCIENCE. 



889 



electrolytes. Accordingly, instead of ex- 

 pressing the concentration in percentage of 

 the solute, Kohlrausch uses ' molecular num- 

 bers. ' The molecular munber of a solution 

 is the quantity, in grams, of the solute con- 

 tained in a liter of the solution divided 

 by the equivalent of the solute. Dividing 

 the conductivity of a solution by its molec- 

 ular number gives its molecular conduc- 

 tivity. It will be seen that 'molecular' is 

 not used here in its ordinary chemical sense, 

 but as the meaning is quite distinctly stated 

 no confusion need arise. Kohlrausch 

 showed that the molecular conductivity in- 

 creases as the solution becomes more dilute, 

 and with extreme dilution approaches a 

 constant value. 



I now show an experiment to illustrate 

 this. 



The apparatus* consists of an electro- 

 lytic cell in the form of a tall rectangular 

 trough, the back and front being broad 

 plates of glass, while the sides are composed 

 of narrow strips of wood completely lined 

 with silver-foil. The bottom of the cell 

 is made of non-conducting material. The 

 two sheets of silver serve as electrodes, be- 

 ing connected to binding screws by means 

 of external wires. The cell is introduced 

 into a battery circuit along with a gal- 

 vanometer of low resistance. If the cell 

 be filled with pure water there is scarcely 

 an appreciable current transmitted. On 

 removing the water and pouring in 20 c.c. 

 of a 4-normal silver nitrate solution, so as 

 to cover the bottom to a depth of a few mil- 

 limeters, a current passes as indicated by 

 the galvanometer. If pure water be now 

 added in successive portions and the solu- 

 tion stirred after each addition, an increase 

 in the strength of the current is observed, 

 the increase being greatest after the first 

 dilution, and becoming less with each suc- 



* From a paper by Noyes and Blanchard, in the 

 Zeitsohrift fur physikalische Chemie, XXXVI., p. 

 9.(1901). 



ceeding dilution, so that a maximum is ap- 

 proached. In this experiment the distance 

 between the electrodes is constant, and the 

 area of the electrodes and of the cross-sec- 

 tion of the conducting solution is propor- 

 tional to the volume of the solution, and 

 the quantity of the salt is constant; there- 

 fore any change in the strength of the cur- 

 rent means a corresponding change in the 

 molecular conductivity of the dissolved 

 salt. The molecular conductivity, there- 

 fore, increases with the dilution, and 

 asymptotically approaches a maximum. 



I cannot here enter into a description of 

 the great experimental difficulties con- 

 nected with the determination of the con- 

 ductivity of extremely dilute solutions, but 

 I may refer to one of them, namely, the 

 small but variable conductivity of the water 

 used in preparing the solutions. There 

 seems now to be no doubt that water is in 

 itself an electrolyte. But the purest water 

 that has been obtained has a conductivity 

 of only about 10—^° as compared with that 

 of mercury as unit. The minutest traces 

 of salts greatly increase the conductivity, 

 so that ordinary distilled water has a con- 

 ductivity of 3 X 10"^° or more. With solu- 

 tions of moderate dilution the variation of 

 this very small quantity is of little conse- 

 quence, but with extremely dilute solutions 

 the conductivity to be measured is of the 

 same order as that of the water. 



For our present purpose the most im- 

 portant conclusion drawn by Kohlrausch 

 from his observations is his law of the in- 

 dependent rate of motion of the ions in 

 dilute solutions. The rate of motion of 

 any ion towards the electrode depends on 

 the gradient of potential. But Kohlrausch 

 shows that the rate of motion of each ion 

 in dilute solution is proportional to a num- 

 ber, the same whatever be the other ion of 

 the electrolyte. Thus the rate at which 

 the cation K moves towards the cathode 

 in dilute solution is the same in solutions 



