Page:Elementary Text-book of Physics (Anthony, 1897).djvu/348

334 assumption that in this case the dissociation is not complete; the ratio of the molecular conductivity found in such cases to the molecular conductivity at very great dilutions, in which case the dissociation is assumed to be complete, is taken as the measure of the dissociation in the solution. A similar theory of partial dissociation was assumed to account for the departures from the normal laws of osmotic pressure (§§ 94, 95), of the lowering of the freezing-point (§ 197), and of the lowering of vapor pressure (§ 204).

The agreement between the conclusions reached by these entirely independent methods with regard to the extent of dissociation is strong evideuce in favor of the hypothesis upon which the calculations are based. Starting with the same hypothesis, other relations have been theoretically discovered among the physical properties of solutions which have been confirmed by experiment. The dissociation theory of solution and of electrolysis is not yet fully established, but it furnishes by far the most satisfactory explanation of the nature and behavior of solutions.

286. Voltaic Cells.—From the discussion given in § 277 it is obvious that, if an arrangement be made, in a circuit, of substances capable of uniting chemically and such as would result from electrolysis, there will result an electromotive force in such a sense as to oppose the current which would eSect the electrolysis. If, then, the electrodes of an electrolytic cell in which this electromotive force exists be joined by a wire, a current will be set up through the wire in the opposite direction to the one which would continue the electrolysis, and the ions at the electrodes will recombine to form the electrolyte. There is thus formed an independent source of current, the voltaic cell. The electrode in connection with the electro-negative ion is called the positive pole, and that in connection with the electro-positive ion the negative pole.

Thus, if after the electrolysis of water in a voltameter, in which the gases are collected separately in tubes over platinum electrodes, the electrodes be joined by a wire, a current will be set up in it, and the gases will gradually, and at last totally, disappear, and the current will cease. The current which decomposes the water is