Valency of the Radio-elements. 587 



some of an experimental and some o£ a theoretical nature; 

 and from a consideration of the results obtained, it appeared 

 necessary that a new and careful determination should be 

 made of the diffusion constants of thorium B and radium E, 

 for which the above method furnished a value of the valency 

 too small by one unit. The possibility of such a result has 

 already been discussed in the former papers*. 



The following paper contains also an account of the deter- 

 mination of the valency of actinium, and of the measurement 

 of the degree of dissociation of lead chloride from diffusion 

 data, and of some experiments on the diffusion of radio- 

 elements in a colloidal state, together with a discussion on 

 the possibility of separating chemically identical elements by 

 means of diffusion. 



The Diffusion of the Chloride of Lead and the Chloride 

 of Thorium B. 



To explain the behaviour of thorium B, the diffusion 

 constants of lead chloride, of thorium B, and of thorium B 

 mixed with lead chloride were measured, since it is supposed 

 that thorium B and lead are chemically non-separable 

 elements. The apparatus was identical with that described 

 in former papers f. The determination of the diffusion 

 velocity of lead chloride was made by rinding the lead 

 chloride content of thin layers which were evaporated to 

 dryness after the interruption of the experiment, while that 

 of thorium B was made by the comparison of the activities 

 of similar layers. In the case of the simultaneous diffusion 

 of thorium B and lead, such a small part of the solution was 

 evaporated that the a rays suffered no measurable absorption 

 in passing the layer of lead chloride. The results obtained 

 are given below. D is the diffusion constant expressed in 

 sq. cm. per day, 0'57 is the constant of the apparatus, the 

 first factor in the denominator gives the time of diffusion in 

 days and the second is taken from Stephanas tables ; it is a 

 number which is a function of the ratio of the concentrations 

 of the final and initial layers, and has come to be suitably 

 referred to as " Stephanas number." I. is the diffusion 

 constant calculated from the uppermost layer, II. that cal- 

 culated from the second layer, and so on. The third layer, 

 as is well known, is not suitable for the calculation of the 

 diffusion constant %. 



* Hevesy, loc. cit. 

 t Hevesy, loc. cit. 



% Sv. Arrhenius, Zeit. phys. Chem. x. p. 53 (1892) ; W. Kaualkiv, 

 Wied. Ann. lii. p. 183 (1894). 



