58 G Dr. Gr. Hevesy on the Di fusion and 



i Q =l'25 amp. and without the external inductance the 

 maximum spark-length (C 1 = , 05 mfd.) was 2*81 cm.; with 

 the external inductance and the same primary current the 

 longest spark was 5'36 cm., and the most effective capacity 

 for this case was '2 mfd. 



The experimental results therefore agree with the theory 

 in indicating that the most effective adjustment of the 

 system (from the point of view of spark-length) is that in 

 which n 2 = 'd?ii, and U is at its maximum. It is probable 

 that most induction-coils have coefficients of coupling which 

 are greater than *8, and in such cases the method here 

 described may be used for making the coil give a longer 

 spark with a given primary current, or a given spark-length 

 with a smaller current. 



The actual values of the secondary potential can only be 

 calculated when the effective resistances are determined for 

 each case, but it is not probable that these latter results 

 would be greatly modified by their introduction. On the 

 whole the evidence tends to show that the theory described 

 in this and the two former papers is sufficient for the 

 calculation of the secondary potential of an induction-coil 

 and the most effective value of the primary capacity. 



Bangor, February 1914. 



LXVI. The Diffusion and Valency of the Radio-elements. 

 By G. Hevesy, Ph.D., Hon. Research Fellow in Physics, 

 Manchester University *, 



IN 1912 the author undertook the direct determination of 

 the valency of the radio-elements by the measurement 

 of the velocity with which their ions diffuse in dilute acid 

 solution f. Since the diffusion velocity of the ions depends 

 only on their mobility and on their valency, and since the 

 first of these quantities can only vary between narrow limits, 

 the magnitude of the diffusion constant of the respective 

 ions furnishes a direct measure of their valency, i. e. the 

 number of charges which they carry. 



Since that time the chemistry of the radio-elements has 

 been examined in detail in a number of excellent researches J, 



* Communicated bv Sir E. Rutherford, F.R.S. 



t Hevesv, Phil. Mag. [6] xxv. p. 390 (1913) ; Phys. Zeit. xiv. p. 50 

 (1913). 



t Fleck, Journ. Chem. Soc. ciii. pp. 394 &1052 (1913); Fajans and Beer, 

 Die Natunvissenschaften, i. p. 388 (1913) ; W. Metzner, Ber. d. Deutsch. 

 Chem. Ges. xlvi. p. 979 (1913) ; Russell, Chem. News,cvii. p. 49 (1913) ; 

 Fajans, Phys. Zeit. xiv. p. 131 (1913) ; Ber. d. Deutsch. Chem. Ges. xlvi. 

 p. 422 (1913) ; Soddy, Chem. News, cvii. p. 97 (1913). 



