152 CONDUCTIVITY OF ELECTROLYTES. chap.
ions are being constantly changed. For example, if half of the molecules of an electrolyte are dissociated, then during the first half of a definite interval of time all the ions are to be regarded as combined, and during the next half they are free. Consequently, although the fall of potential only acts on the free ions in every infinitely small interval of time, yet in a finite time all the molecules will be similarly n^^ved, inasmuch as during the first half of this time they migrate as ions, and in the second half they remain at rest in the undissociated state. The apparent velocity will in this case be only half of the actual. In agreement with this, Whetham (i?i) found a much lower velocity for the hydrogen ions from acetic acid than for the same ions from hydrochloric acid, corresponding with the lower degree of dissociation of acetic acid.
Eecently ionic mobilities and degrees of dissociation in other solvents than water have been determined by VoUmer (J:?), Carrara (^J), Euler QU), Walden (^5), and others.
Diffusion — Besides the electrical, other forces may be active in causing the movement of the ions. Of these the osmotic pressure is the most important. On account of this pressure a phenomenon called diffusion (hydrodiffusion) may
be observed. Consider a solu- tion in a parallel-sided trough of 1 sq. cm. cross section (Fig. 38), and imagine two planes, A and B, perpendicular to the column of liquid, 1 cm. apart. Let the solu- ^^- 38. tion at A he {n + normal
and that at B {n ~ i) normal. The osmotic pressure at A is then greater than that at Bj and if the molecules of dissolved substance are not dissociated the excess of pressure at ^ is equal to the = 23,120 grams per sq. cm. at 0° (compare pp. 26 and 30). This excess of pressure drives the dissolved molecules in the
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