In the first of these ca-es, some ions will tend to precipitate themselves upon the metal bar, just as sonic of the vapor of a liquid would tend to condense, if its pressure exceeded the vapor tension of the liquid : and the force driving the ions out of solution will evidently be the excess of their osmotic pressure over the solution tension of the metal. As soon, however, as a single ion is precipitated out, the metal bar is rendered elect•o-positive, while the solution is rendered eleetro-negative by the newly created excess of eleetro-negative ions; for before the immersion of the metal bar the electropositive and eleetro-negative ions in solution are pre cisely equivalent. (See Dissoct.knox.) But this means that a difference of potential will have been created between the metal bar and the solution. henceforward, every eleetro-posi tive ion driven toward the metal liar by the exces sive osmotic pressure will encounter a double force driving it back into solution, viz. the electrostatie repulsion of the positive bar and the electrostatic attraction of the negative solu tion. Finally, when a certain number of ions have been precipitated out, the double electro static counter-force will have become equal to the excess of osmotic pressure, and then, equi librium ensuing, the maximum difference of po tential possible under the given conditions will bane been established between the metal and the solution. This will obviously be the greater, the greater the osmotic pressure (i.e. the con centration of ions in the solution), and the less the specific solution-tension of the metal. Pa-sing now to the second of three possible case:. viz. the case in which the osmotic pressure of the metal lic ions is less than the specific of the metal, we see that at this ease molecules of the metal will enter the solution in the form of positive ions, rendering the solution elect•o positive and leaving the metal bar electro-nega live. In tins case, too, an electrostatic counter force will then come into play, and the differ ence of potential at the surface of contact will be maximum when this force has become equal to the difference between the solution-tension of the metal and the osmotic pressure of ions in the solution. The difference of potential will ob viously be the greater, the greater the solution tension of the metal and the less the osmotic pressure of its ions in the solution. Finally, in the third case, viz. the case in whh-11 the osmotic pressure of the metallic ions exactly equals the solution-tension of the metal, neither metal will dissolve nor ions precipitate, and consequently no difference of potential will exist at the surface of contact of the metal and the solution.
Theprinciples just developed permit of gaining a clear insight into the mechanism of eleet•o-ehem ical action. Take, for instance, again the Daniell cell, which consists, as we have seen. of a con centrated solution of zinc sulphate and a concen trated solution of copper sulphate, separated from each other by a porous partition, with a bar of metallic zinc in the former solution and a bar of metallic copper in the latter (see Fig.).
It will be seen in a later section of this article that the solution-tension of zinc is very great, that of copper very small. Some zinc will there fore enter the solution in the state of positive ions, which will render the zinc solution electro positive and leave the zinc bar eleetro-negative. On the other hand, a number of copper ions will join the copper bar in the state of metallic cop per, wndering the liar elect•o-positive and leav ing the copper sulphate solution electro-nega tive. Equilibrium will ensue when the excess of solution-tension in the case of zinc and the excess of osmotic pressure in the ease of copper are exactly counterbalanced by the electrostati• forces as explained above. Now, if we should then connect the Iwo bars by means of a metal wire, a flow of electricity t positive electricity from copper to zinc, negative from tine to cop per) take place between the two bars. and thus the equilibrium would be destroyed. Freed iroin the electrostatic attraction, the excessive zinc ions of the zinc sulphate solution would migrate toward the excessive n•gative ions of the copper sulphate solution. At the same time, more zinc. driven by the unchecked solution tension, would go into solution, and more cop per ions, driven by the unchecked osmotic pres sure. would be precipitated upon the Nipper bar. If we should break the connection between the bars, the former equilibrium would soon be re established, and chemical action WOLIN cease. But as long as the connection exists, zinc goes into solution, copper is precipitated, and a current of electricity passes through the connecting wire; we say. "electro-clumical action is going on."
Suppose now that the copper sulphate solu tion of the Daniell cell was replaeed by a solu tion which could take up molecules from the copper bar. but in which those molecules, instead of being transformed into ions, would form a undissoeiated compound with some other ingredient present. The osmotic pressure of copper ions would thus be extremely slight and might be exceeded by the electrolytic solu tion-tension of copper. in spite of the latter being comparatively small. It is, therefore, easy to see that under certain conditions it might be possible to reverse the electro-chemical process of the cell. As a matter of fact, this is precisely what happens when the copper bar of a Daniell cell is immersed into a concentrated solution of the cyanide of potassium. On closing the circuit, copper then goes into solution, zinc is precipi tated, and the direction of the current is reversed. This and similar phenomena were formerly de s•ribed as 'anomalous,' because they seemed to contradict the assumption that zinc has a greater 'affinity' for acids than copper. Substi tuting the definite concept 'solution-tension' for the vague concept 'affinity,' and adding the factor of the 'osmotic pressure of ions,' we ex perience no difficulty in understanding effects like the reversion of current in the Daniell cell by potassium cyanide. and many other phe nomena. And it must be observed that the osmotic pressure of ions is a measurable quan tity I see SoLuTiox : DISSOCIATION). and that the solution-tension of metals, too, has now been calculated from directly measurable quantities (see further below).
Closely allied to the eleetrowhemical action of voltaic cells is the immediate precipitation of One metal by another. The precipitation of copper by iron from solutions of copper salts, and the precipitation of finely divided ('molecu lar') silver by zinc, are familiar examples of this phenomenon. the cause of which is almost self-evidd•ut in the light of our theory. When a bar of iron, for example, is immersed in a solu tion of copper sulphate, the solution-tension of he iron causes t he appearance of some iron ions, and hence the formation of an excess of positive over negative electricity in the solution; the electrostatic relmlsion t he metallic ions then tends to drive both iron ions and copper ions of solution; but the solution tension of iron is very great, and so the iron ions remain in solution; on the other hand. the slight solution-tension of copper is readily over. come by the force of electrostatic repulsion. and so copper ions are driven out and appear in the state of metallic copper. An analogous exchange takes place when a metal dissolves in an aqueous solution of sonic acid, the solution-tension of hydrogen being overcome by the electrostatic re pulsion between the ions of hydrogen and those of the metal. Only here the external pressure of hydrogen gas conics into play: for the solution tension of a gas naturally depends upon its pres sure. We have seen above that a voltaic cell may be obtained by using a dilute a(-id and two quantities of hydrogen gas under different pressures; the two quantities of hydrogen act like two different metals because, owing to difference of pressure, their solution-tensions are different. Further, we see that while for certain pressures of gen the solution-tension of a given metal may be greater, for other pressures of hydrogen the solo-• tion-tension of the metal may, on the contrary, be less, than the solution-tension of hydrogen. In other words• if we should place in a vessel filled with hydrogen under sufficient pressure the solution of sonic salt, the metal might be precipitated out of flue hydrogen. As a' matter of fact, this curious phenomenon has been observed in the case of several metals. If in a vessel containing hydrogen under a pressure of IS atmospheres (270 pounds per square inch), metallic zinc is brought into contact with a solution containing per liter grams of sulphuric acid and 210 grams of zinc sulphate. no action takes place. If the pressure is slightly diminished, zinc goes into solution and hydro gen is evolved. lf. on the contrary. the pressure is somewhat increased. hydrogen goes into solu tion and zinc is precipitated. That the con centration of hydrogen gas (i.e. the amount com pressed within unit volume) plays the determining role in this phenomenon is clearly evident: and so this and similar phenomena, too• go to show the inadequacy of older chemical theory. which recognized (in a vague qualitative way) only the 'affinity' factor; and, leaving out of account the factor of concentration, 'explained' the dissolu tion of metals in acids by the assumption that metals have greater affinity for acid radicles than hydrogen.