i8o PRINCIPLES OF GENERAL PHYSIOLOGY 



23-9 x 10 6 dynes. The osmotic force is therefore 2 x 10 5 times less than the 

 electrostatic force preventing diffusion ; in other words, the latter is 200,000 times 

 as strong. We see then how an extraordinarily minute excess of Cl over Na ions 

 would suffice to prevent any further diffusion. If one ion moves on, the opposite 

 one must follow it at an infinitesimal distance. 



A more difficult question arises from considerations of energetics. We know 

 that sodium and chlorine combine with the evolution of heat in considerable amount, 

 so that, in order to separate them as ions when the compound is dissolved in water, 

 a corresponding amount of energy must be supplied from some source. The fact 

 that ions are hydrated seems to offer a possibility, if we regard the hydrates as 

 chemical compounds, formed with evolution of heat. 



Bousfield and Lowry (1907, p. 125) suggested that the affinity of the "ionic nucleus" for 

 water is the main source of the energy required. Further evidence for this view is given by 

 Bousfield (1912, p. 149). The argument may be put very briefly thus : Atoms, being composed 

 of large numbers of more minute bodies or corpuscles, may be regarded as compressible. 

 The heat of formation of a compound is found to be approximately equal to the sum of 

 certain "calorific constants" of the components, together with 0'875 <5V, where 5V is the 

 change of atomic volume which takes place. From this fact, the internal energy of an atom 

 is to be regarded as the sum of the kinetic energy of the corpuscles, and of the potential energy 

 due to their mutual attraction. The factor, 0'875 5V, therefore represents the change in 

 internal energy of the atoms due to their change of volume on combination. How compression 

 or contraction diminishes the internal energy of an atom by approximation of mutually 

 attractive corpuscles may be found on p. 151 of the original paper. Applying these considera- 

 tions to the estimation of the components of the heat of formation of solid, liquid, and ionic 

 molecules, it is found that 5V is considerably greater in the ionic state than in that of solid 

 or liquid ; thus, the value for KC1 in the ionic state is 42 '7, for the solid state, 32 '5.. That is, 

 the contraction which takes place on combination in the ionic state is greater than that in 

 the solid state, and may well be the source of the energy required for electrolytic dissociation. 



Larmor (1908, p. 37) states the possibility that "internal potential e,nergy is released 

 owing to the ions entering into relations of closer affinity with the solvent. There is, of 

 course, no doubt as to the capacity of molecular forces to afford the energy required, hut the 

 question still remains, what should cause them to give it up for the purpose of dissociating 

 dissolved salts ? We may say that the affinity of an ion for water is greater than that which 

 it has for an opposite ion, but is this any more than a re-statement of the problem in another 

 form? 



A further difficulty that has been put forward is this. Granting that, by some 

 means or other, the ions have been separated, what is to prevent the opposite 

 electrical charges from neutralising one another with production of the salt again ? 

 We have seen that an analogous process does actually occur in the mutual 

 precipitation of oppositely charged colloids. The answer is probably bound up with 

 that to the previous question. The forces which caused the dissociation are pre- 

 sumably continually active in preventing recombination. 



There is no doubt that the dielectric constant of the solvent is, in some way, 

 intimately involved in the process. Tt is not an easy matter to picture the way in 

 which it acts, but the following points may possibly be of assistance to the reader. 



Two oppositely charged bodies, as is well known, attract one another with a certain force, 

 which can be measured. It might be supposed that this force would be independent of the 

 substance between the two bodies, provided that it be an insulator. But this is not so, as 

 Faraday found. Suppose that air is the insulator between the two bodies, and that they have 

 such a charge, and are at such a distance from one another, that the force tending to bring 

 them together is equal to the weight of 10 g. Now put petroleum in place of air, it is found 

 that the force is only 10/2 - 2, and, if castor oil be used, it is only 10/4 '3. The denominators of 

 these fractions are known as the "dielectric constants " of the liquids, and they play a part in 

 other connections. The capacity of a condenser, for instance, is greater, the greater the 

 dielectric constant of the material between the plates ; that of mica being 8, the reason for 

 using this material instead of paraffin, with a dielectric constant of only 2'3, is obvious. 

 According to the modern theory of electrons, the dielectric constant is the greater, the larger 

 the number of electrons present in a given space of the substance. These act as conducting 

 particles and are surrounded by an insulating substance of very special properties, the 

 luminiferous ether. In the course of their propagation through a non-conductor, electric 

 forces must exert an action on these electrons ; so that it can be understood why, the more of 

 them there are, the greater is the obstruction to the forces. The connection between 

 electricity and light, as the reader will remember, was worked out by Clerk Maxwell, and, in 

 the present connection, it is of interest to recall the fact that the dielectric constant of a 

 substance is identical with the square of its refractive index, as calculated for light of very 

 long wave length or electric waves (Maxwdr* Law). 



