432 On the Theory of the Electrolytic Solution- Pressure. 



nitude 10 -8 cm.*, then the production of a pressure so low 

 is impossible ; for pressure is a statistical effect due to the 

 impact of numerous molecules ; and in order to give such a 

 pressure the solution would need to contain only one or two 

 molecules of palladium in a space the size of the earth. 

 Hence II can here have a real meaning only if the metal is 

 very greatly more divisible than the ordinary molecular theory 

 assumes. 



There is, however, another objection, not involving the 

 molecular theory at all, which may be put as follows. 



If an electrical double layer is formed between the metal 

 plate and the ions in solution near it, a tension is set up 

 between the two. The amount of that tension may be calcu- 

 lated by assuming (i.) that the electricity is distributed in 

 two layers of uniform surface-density; (ii.) that the distance 

 between the two layers is small compared with their extent. 

 The former of these is the more nearly true the smaller the 

 actual particles (ions) on which the electric charge occurs : 

 the second assumption may presumably be adopted on any 

 theory of the phenomenon, since the surface of contact may 

 be square centimetres in extent, while the distance between 

 the layers is indefinitely small. Calling a the surface-density, 

 the tension produced is 



27rer 2 



where D = dielectric constant of the medium. Now to pro- 

 duce a surface-density a we require, per square centimetre of 

 surface, x = a/e gram equivalents of metal to go into solution: 

 so that the tension produced is 



27re 9 # 2 



Applying this result to zinc, and neglecting the trifling 

 osmotic pressure by comparison with the enormous solution- 

 pressure, we have 



e =96540 coulombs = 9654 x 3 x 10 10 static units, 



D = 80 (for water), 



II = 9 , 9 x 10 18 atmos = 10 25 dynes per sq. cm., 



and 



V2^r7 2 =0 ' 039 g m - e( l uiv -> 

 = 1*27 gm. 



* See Thomson and Tait, ' Natural Philosophy,' vol. ii. p. 495 

 (2nd edit.). 



