184 Mr. W. C. Dampier Whetham. [May 30, 



If the potential-gradient at the junction is dV/dx, we hare 



dV/dx = -yr/A, 



where 7 represents the total current, r the specific resistance of the 

 solution, and A the area of cross-section of the tube. 



If v is the observed velocity, the specific velocity for unit potential 

 gradient is given by 



v _ vA. 



~ dVjdx 77- 



A is determined by filling a known length of the tube with water or 

 mercury, 7 is read off on a galvanometer previously graduated by 

 means of a Daniell cell and a box of resistance coils, and r is de- 

 termined by Kohlrausch's method of a Wheatstone's bridge with 

 alternating currents. 



The solutions must be of nearly equal conductivities, so that a 

 mean value of r may be taken. This is the more important because, 

 unlike the colour-boundary method, the formation of a precipitate is 

 an irreversible process. Measurements cannot, therefore, be made 

 with the current flowing in both directions, which, in the former 

 paper, was shown to get rid of the disturbing effect of any remaining 

 difference in conductivity. All that can be done is to choose solu- 

 tions whose conductivities are very nearly equal, so that the un- 

 certainty which must appear in the result shall be, at all events, as 

 small as possible. 



The apparatus was immersed in a water bath, and the results all 

 corrected to a temperature of 18 C., in order that they might be 

 comparable with Kohlransch's calculated values. 



The following results were obtained : 



Barium. Solutions used : decinormal barium chloride and sodium 

 chloride, a little sodium sulphate being added to the latter. 



Temperature, 15*8. Mean conductivity at 15*8 in reciprocals of 

 legal ohms, 9'60 X 10~ 3 . Mean current, 1-08/131 ampere. Area of 

 cross-section of tube, O430 sq. cm. Mean velocity of precipitate, 

 0*446 cm. in 10 minutes. 



= 0*000372 cm. per sec. 



The temperature coefficient was found to be 2*5 per cent, per degree, 

 so that we get for the specific ionic velocity of the barium ion, 

 travelling through a decinormal solution of barium chloride in solid 

 agar jelly at a temperature of 18, 



flBa = 0*000393 cm. per sec. 



For an aqueous solution of this strength Kohlrausch gives {' Wied. 

 Ann.,' vol. 50, p. 385) 



UBa = 0-000366 cm. per sec. 



