October 9. 1896.] 



SCIENCE. 



517 



Electrolytes give greater values than non- 

 electrolytes. The effect is as though there 

 was present a greater number of molecules 

 than indicated by the amount of substance 

 dissolved. Binary compounds in dilute 

 solutions, give nearly twice the numerical 

 values found in solutions of non-electro- 

 lytes. 



From these experimental results the 

 theory of dissociation of electrolytes by 

 solution and the electrolytic conduction by 

 directing the migrating ions seems very 

 strong. 



Chemical evidence can not be reviewed 

 here ; the evidence is, however, fully as strong 

 or even stronger than that considered. 



Reversing the phenomena of electrolytic 

 decomposition, ISTernst applies these con- 

 cepts to the battery problem. We will apply 

 the theory to one or two simple cases only. 



When two solutions of the same electro- 

 lyte are brought in contact electrical differ- 

 ences manifest themselves. The mere state- 

 ment, that the cause for such difference in po- 

 tential is contact, is unsatisfactory. Nernst 

 reasons in this wise : The ions in virtue of 

 osmotic pressure will diffuse; migrating 

 with different velocities, as shown by Hit- 

 torf, they will diffuse at different rates; 

 there will result, therefore, an excess of 

 anions in the one solution and kathions in 

 the other; the ions possessing characteris- 

 tic electric charges, there will be found an 

 excess of positive electricity in the one solu- 

 tion and an accumulation of negative elec- 

 tricity in the other solution. If an indiffer- 

 ent electrode be immersed in each solution, 

 connected through a conducting circuit, 

 there must result a current. We have here 

 a battery giving an electrical current as a 

 result of osmotic pressure. The necessary 

 conditions for the production of the current 

 are, that the ions have different velocities 

 and exert osmotic pressure ; that in solu- 

 tion we have the molecules dissociated or 

 ionized ; that the energy associated if current 



flows, is less at the close of the operation 

 than at the beginning. 



Nernst and Helmholtz calculated the 

 electromotive force for such cells from the 

 experimental data of osmotic pressure and 

 ionic velocities, obtaining results agreeing 

 very well with electrical measurements. 



It would, perhaps, be expected from the 

 above that the current would continue until 

 the concentration had become uniform; 

 such, however, will not be the case in the 

 battery just described, for the kathions at 

 the kathode and the anions at the anode 

 will now develop electrostatic attractions 

 which can not be overcome by the electro- 

 motive force of the battery. 



Modifying the cell so that, in place of 

 indifferent electrodes, we use electrodes of 

 the metal, the salt solutions of which sur- 

 round it, a battery results which will con- 

 tinue in operation until uniform concentra- 

 tion results. In this case the kathions 

 will, at the kathode, by giving up their 

 electric charges, become metallic, the elec- 

 trode becoming positively charged ; at the 

 anode metallic particles will go into solu- 

 tion as kathions. This ionization involves 

 energy changes. The kathions there pro- 

 duced, carrying positive charges with them, 

 leave the electrode negatively charged. 



In a battery, such as described, three 

 differences of potential result; one at the 

 surfaces of the solutions differing in concen- 

 tration, and a difference of potential be- 

 tween the surfaces of the electrodes and 

 the solution, the first having been explained 

 as originating from osmotic pressure. 

 Nernst explains the latter as resulting from 

 a solution pressure. Just as a liquid 

 evaporates from the surface until the vapor 

 pressure becomes equal to the vaporization 

 tension, so in solution will a salt dissolve 

 until the osmotic pressure becomes equal to 

 the solution tension of the salt. In the 

 same way Nernst supposes that each metal 

 has a tendency to convert atoms of the 



