aqueous solution of rubidium and caesium chlorides. 383 



Five cubic centimeters of a warm 47o solution of well washed 

 gelatine* were introduced by a pipette and allowed to solidify. 

 The cell was immersed in a water bath kept at 18° C. The 

 resistance of the jelly was determined by the telephone method, 

 using a post-oflSce resistance box ; the alternating current was led 

 off from the primary of a small induction coil, the secondary being 

 connected in series with a 10 c.p. lamp to the 50 volt, 90 '^, house 

 supply. ^ 



The resistance of the jelly was not affected by standing m 

 contact with distilled water for over three hours : it is therefore 

 certain that the progressive changes in conductivity observed when 

 salt solutions are substituted for the water are not complicated 

 by the diffusion of impurities away from the gelatine. The salt 

 solutions used were deci-normal. 10 c.c. of the solution were 

 poured on the jelly, filling the cell to the level indicated in the 

 figure, and the resistance of the gelatine between the electrodes 

 was measured, usually at intervals of half an hour, for the next 

 eight or ten hours. 



The specific conductivity was obtained for each reading by 

 dividing the constant for the cell (•643) by the resistance in ohms. 

 From this the specific conductivity of the gelatine at the beginning 

 of the experiment was deducted, the result indicating the specific 

 conductivity due to the salt. The molecular concentration of the 

 salt corresponding to each of these values was read off from curves 

 constructed from the figures given by Kohlrausch-f" and his co- 

 workers. At the dilutions with which we are concerned the 

 specific conductivity is very nearly in linear proportion to the con- 

 centration. 



On plotting the concentrations against the times from 

 the beginning of the experiment the curves were found to be at 

 first convex to the base line and towards the end of the experi- 

 ment slightly concave, but in every case over a period of at least 

 two hours their progress was rectilinear. The inclination of this 

 part of the curve differed for each salt. I assumed that during 

 the rectilinear period the rate of rise in concentration would be 

 directly proportional to the velocity of diffusion of the salt. In 

 order to test the validity of this assumption I made experiments 

 with lithium, sodium and potassium chlorides, since the relative 

 rates of diffusion of these substances have been determined already 

 by physicists, using other methods:):. 



* A gelatine jelly of this strength has been shown by Hardy to have the struc- 

 ture of an open network. 



t Quoted in Handhuch der anorg. Chemie, ii. 1, Abegg u. Auerbach, 1908. 

 Also, Landolt-Bornstein's Tabellen, 1903. 



X I have unfortunately been unable to apply the method of calculation, since 

 worked out by Mr Hill, to these observations owing to the neglect of a simple 

 precaution in my first experiments. The solution was introduced at a temperature 



25—2 



