122 



NATURE 



[June 7, 1900 



Another matter with regard to which a greater 

 definiteness seems desirable, even at the expense of some 

 generality, is the theory of the action of the voltaic cell. 



Nernst's theory of the electrolytic " solution-tension " 

 of a solid— solution-pressure is a preferable term — is 

 stated in his own words, but they are vague : 



" We must ascribe," it is said, after a reference to 

 osmotic pressure, " to a dissolving substance in contact 

 with a solvent, similarly, a power of expansion, for here 

 also the molecules are driven into a space in which they 

 exist under a certain pressure. It is evident that every 

 substance will pass into solution until the osmotic partial 

 pressure of the molecules in the solution is equal to the 

 'solution-tension' of the substance" (pp. 231-232). 



We may put the whole theory slightly differently, 

 thus :— 



In the case when a substance is being dissolved in 

 such a way that molecules pass from the solid into the 

 liquid, the pressure rises in consequence of the impacts 

 of these molecules on the walls of the containing vessel ; 

 now when molecules also pass from the liquid into the 

 solid, the " evident " fact is that the steady state is reached 

 when the numbers entering and leaving the liquid are 

 the same. In such a case the osmotic pressure measures 

 the solution-pressure, and no electrical action is involved. 



But now let us suppose that a metal is passing, not in 

 the form of molecules, but in that of ions into water. Each 

 of these ions carries with it a positive charge ; the water 

 therefore tends to become positive, the metal negative, 

 and an electrical double layer is formed over the surface 

 of separation. The charged ions are not free to move 

 throughout the water, but {t.yN escape from the surface ; 

 hence the additional pressure due to the impacts of the 

 metallic ions — the solution-pressure, as it is called — is 

 small. 



Again, let us take the case of a metal, such as copper, 

 in a solution of one of its own salts, say copper sulphate ; 

 here, also, if there were no electrical effects, we might 

 suppose that copper molecules would be deposited out of 

 the sulphate on to the metal, while other molecules would 

 leave the metal ; the steady state would be reached when 

 these two sets of molecules became equal in number, and 

 the osmotic pressure would become— in reality, unless the 

 solution were very weak, would /«// to— the solution pres- 

 sure. But according to the theory, the copper passes as 

 ions which carry with them out of the solution their 

 positive charge ; this they give up to the metal on 

 becoming molecules. And since w-e suppose that, un- 

 less the solution be very weak, the number of copper 

 ions leaving it is, to start with, greater than those enter- 

 ng, the metal becomes positive, the negative ions of the 

 solution are attracted to it, the positive ions driven off, a 

 double layer is again formed ; a difference of electrical 

 potential is established between the metal and the solu- 

 tion—the metal being positive, the solution negative. 



If, however, we consider a metal, such as zinc, which 

 has a high solution pressure when immersed in, say, zinc 

 sulphate, we must suppose that at the start more metallic 

 ions leave the metal than enter it, the solution thus be- 

 comes positive, the metal negative, and the double layer 

 formed is one which tends to prevent the positive metallic 

 ions from leaving the zinc, and is thus opposite to that 

 formed on the copper. 



NO. 1597, VOL. 62] 



In both these cases we must suppose, when the steady 

 state is reached, that the ions leaving the metal leave it 

 under the solution pressure of the metal in the liquid. 

 This may be seen as follows : If there were no electrical 

 force called into action, the pressure would go on changing 

 in the liquid up to the solution pressure, when the number 

 of metallic ions leaving the surface would balance those 

 entering. 



Thus the solution pressure measures the whole amount 

 of momentum which the ions of the metal tend to transfer 

 per second aero ss unit area of the surface. Now according 

 to the theory this momentum depends on the metal only, 

 and the tendency to transfer momentum remains the 

 same, however the transfer be stopped ; in reality, the 

 electrical forces acting across the double layer stop it, 

 not the opposing momentum of the liquid ions, and the 

 pressure exerted by these electrical forces must be there- 

 fore equal to the solution pressure of the solid, i.e. when 

 a current is flowing the positive ions start from the metal 

 at the solution pressure of the metal, and become, when 

 in the solution, ions at the osmotic pressure of the liquid. 



Now, however, let us suppose that a piece of copper is 

 connected to the zinc, the two being dipped into zinc sul- 

 phate ; and suppose further, for simplicity, that there is no 

 action at the interfaces zinc-copper or copper-liquid, then 

 negative electricity from the zinc passes over to the 

 copper through the zinc-copper junction, attracting to 

 itself the positive ions in the solution and destroying the 

 double layer at the zinc-liquid junction ; thus a current of 

 positive electricity passes through the solution from zinc 

 to copper. The source of the E.M.F. is at the zinc-liquid 

 junction, arising from the fact that more zinc ions pass 

 from the zinc into the solution than from the solution into 

 the zinc ; or, as Nernst would put it, that the solution 

 pressure of the zinc is greater than the osmotic pressure 

 of the liquid. In reality, of course, there may be actions 

 at both the other junctions similar in character to that 

 which we have supposed to go on at the junction of the 

 zinc and the liquid, and the resultant E.M.F. depends 

 on all of these.^ 



In this simple case the energy of the cell is obtained 

 from the passing of the zinc ions from the saturation 

 pressure of the zinc to the osmotic pressure of the liquid, 

 and we obtain at once Nernst's expression for the electro- 

 motive force, varying as RT log.e P/A where P is the 

 saturation pressure, p the osmotic pressure. 



But an article which started as a notice of Mr. Jones' 

 most useful book is in danger of becoming a dissertation 

 on the seat of the electromotive force of a voltaic c^U. a 

 result to be avoided. R. T. G. 



MESOZOA AND ENANTIOZOA. 

 Traite de Zoologie Concrete. T, ii. r* partie. Mdsozo- 

 aires — Spongiaires. By Yves Delage and Edgard 

 Herouard. Pp. ix -F 244, (Paris : C. Reinwald, 1899.) 



AS might have been anticipated, this part of the mas- 

 sive " Traite de Zoologie," which is now in course 

 of publication, contains matter of exceptional interest. 

 One-fifth of the present issue is devoted to the Mesozoa, 



1 A reference should be made to Prof. Lodge's article in the May number 

 of the Philosophical Magazine, which has appeared since the above wa 

 written. 



