JONE 28, 1918] 



SCIENCE 



637 



the opposite direction to that supposed in Pro- 

 fessor Sanford's explanation, because increase 

 in ion content must increase the inductivity of 

 the solution, as will appear from the following 

 consideration : 



As ions pass from the metal into the solu- 

 tion, the changing composition of the mixture 

 is accompanied by an increase in its density. 

 The density, d, of a solution of any given con- 

 centration is related to its index of refraction 

 of light, n, approximately as shown by the 

 equation, (n — l)/d = E, the specific refrac- 

 tive power, a constant. A more concentrated 

 solution, i. e., a different proportion of the 

 same components, which has a grreater value 

 for d, will also have a greater value for n, since 

 the values of these physical properties depend 

 additively upon the values of the same proper- 

 ties of the components. It would not be proper 

 to substitute for n in the above expression the 

 square root of the dielectric constant, as the 

 electromagnetic theory might suggest, because 

 the latter relationship is not capable of ex- 

 perimental test under the conditions for which 

 the former is found to hold. But while the 

 exact form of the function may be unknown, 

 there can be no doubt that when refractive in- 

 dex increases as in the above case, the induc- 

 tivity must increase also. 



Applying this to a concentration cell, on the 

 dilute side the inductivity of the solution is 

 increasing, and this increment in the induc- 

 tivity favors the further solution of the metal, 

 but the osmotic pressxire of the metallic ions 

 is also increasing, and this increment opposes 

 the further solution of the metal. Solution 

 pressure, the predominating force on the dilute 

 side, is aided by inductivity, and these together 

 constitute a growing force — opposed, however, 

 by a more rapidly growing force, osmotic pres- 

 sure. In the more concentrated solution 

 aroimd the other electrode, we have an initially 

 greater inductivity which is decreasing as 

 metal ions are discharged and deposited, and 

 this decrease of inductivity favors the depo- 

 sition (or opposes the solution) of the metal ; 

 but the osmotic pressure of the metallic ions 

 is decreasing also, and this decrease opposes 

 the deposition. On this side, solution pressure 



is aided by a relatively large but decreasing 

 inductivity, and combined they constitute a 

 diminishing force which is initially weaker 

 than the opposing osmotic pressure, but 

 stronger than the corresponding solution pres- 

 sure of the other electrode. All of the pressure 

 differences in the cell owe their existence to the 

 difference in concentrations of the solutions, 

 and all reach equilibrium when the concentra- 

 tions become equal. 



In formulating the total combined effects 

 on both sides of the cell the inductivity effect 

 is either added to the solution pressure or 

 subtracted from the osmotic pressure of the 

 cations in solution. We are not so much con- 

 cerned here with the value of the ratio we call 

 inductivity or its nature, as with its effect, 

 which is a pressure. Let us call this the 

 modulus i, then the familiar equation becomes 



nF \ p, J nF \ piil+i,)J 



In this we have assimied, after all, that fun- 

 damentally the solution pressure is constant, 

 but that there is a difference in effective 

 solution pressure due to difference in induc- 

 tivity. This seems reasonable where we are 

 dealing with the same solvent as in a simple 

 concentration cell : here tlie differences in in- 

 ductivity are probably small. Would this 

 equation suffice for different solvents in which 

 t, and i^ are unrelated, or must we still keep 

 P, and P„ distinct and find some further cause 

 for a difference in solution tension of the same 

 metal ? 



A series of inductivity measurements for 

 varying concentrations of, say zinc sulphate, 

 in water, with measurements of electromotive 

 force of elements composed of zinc In the same 

 concentrations of the salt, might lead to a 

 clearer knowledge of the magnitude of solution 

 tension, and might even throw some light on 

 tlic as yet imknown forces whose resultant we 

 call dissociating power. 



In conclusion, allow me to say that this is 

 not written in a spirit of controversy, but in 

 order to place a little of our existing knowl- 



