60 ION I SAT I ON 



It is a measure of the obstruction produced as described above, i.e. 



it measures the capacity of the mediuui to act as an insulator. When 



the distance /• between the two charged bodies is increased so that 



ee' 

 Kr^ is very large compared with ee', the force t^, becomes ncghgible, 



and the two bodies will cease to attract (if of opposite signs) or 

 repel one another (if of the same sign). Suppose this happens at a 

 distance r^ in a medium with a dielectric constant K^, and at a 

 distance r2 when the medium has a dielectric constant of A'2, 



ee' ee' 



and as the charges e and e' are obviously the same in both instances, 



then ^1^1^ = K^ro^ and /-g = r, v/ t^ . 



From the following Table (VII.) it may be seen that air is 

 arbitrarily taken as having unit dielectric constant, and on this 

 basis water has a dielectric constant of 81-7. 



If the 

 respectively, and substituting values in the equation for /'g, we have 



r. 



I 



ri 



2 = ^'i V SW ^ 9 ^PP^o^- 



That is, the distance between charged ions in water may be nine 

 times less than in air without the one ion exerting an appreciable 

 influence on the other. Thus, when the molecules have been split 

 into their constituent ions, the high dielectric constant of water 

 lowers the probability of their recombination to such an extent 

 that the solution is stable in this dissociated form. 



This does not, however, describe how the splitting of the mole- 

 cules into ions is brought about, nor why some substances are easily 

 and almost completely dissociated at a certain dilution, while 

 others under the same conditions undergo an almost negligible 

 dissociation. Many solutes dissolve in water to give highly 

 dissociated solutions without any great change in the sum total of 



