118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. H. HARTLEY AND H. R. RMKES 393 THE MOBILITIES OF THE ELEMENTARY IONS IN METHYL ALCOHOL. By H.HARTLEY and H. R. RAKES. Received 30th March, 1927. The mobilities of the elementary ions possess a special interest as their structure is known more definitely than that of composite ions such as NO,-, and some evidence as to their relative sizes may be obtained from 11 R. Lorenz, Zoc. cif., pp. 87 and 261.394 MOBILITIES OF IONS IN METHYL ALCOHOL H,O. 1 MeOH. ~- Mg . Ca . Sr . Ba . Zn . Cd . - - their effective ionic radii in the crystal lattice which have been shown to be approximately constant in simple polar assemblages. I n water, as is well known, the ions of the alkali metals and of the halogens, which have the largest radii in the crystal lattice, have the highest mobilities, while all the ions of the divalent metals have roughly the same niobilities in spite of a considerable increase in size with atomic number.These facts are usually explained by the assumption that the ions are hydrated, and that the degree of hydration of the small ions is larger on account of the more intense elec- trical field around them, their mobilities being correspondingly reduced. Washburn's determination of the relative number of water molecules asso- ciated with the ions of the alkali metals affords strong evidence in favour of this view. We have been engaged since 192 I in investigating the conductivities of dilute salt solutions in methyl alcohol, and our results, together with the determination of the transport number of the chlorine ion in hydrogen chloride by Nonhebel and Hartley,l now enable us to compare the infinite dilution values of the mobilities of all the univalent elements and most of the divalent metals in methyl alcohol with those in water at 25' C.The values in Table I. for univalent ions in methyl alcohol are taken from Frazer and Hartley's paper, with the exception of T1+, the rest are from unpublished work by McChlery (TI+), Philbrick (Ca++, Sr++, Ba++), and Ross (Mg++, Zn++, Cd++). I H,O. i MeOH. 1- 53.0 I 57.6 59.8 I 60.0 59'8 1 59'0 64.2 I 60.0 54-2 596 53'5 57'4 - - - I - H . Li . Na . K . . Rb . cs . . 3. : H,O. j MeOH. -- 53'8 I n methyl alcohol as in water the mobilities both of the alkali metals and halogens increase with atomic number, the rise being more regular in methyl alcohol (in water for example the mobility of bromine is slightly greater than that of iodine).The mobilities of silver and thallium are high in both cases, and bear roughly the same relation to caesium in each. The mobilities of the divalent metals are even more nearly equal in methyl alcohol than in water. Thus the general relationships of the ionic mobilities are the same in both solvents, as is seen in the figure, in which the mobili- ties of the ions divided by their valencies are plotted against their atomic numbers. This expresses the relative magnitudes of their mobilities under equivalent conditions, as in the same electrical field the force acting on a divalent ion is twice that on a univalent ion. I n spite, however, of the general agreement the relative mobility of the ions in the two solvents is not constant, as is seen in Table 11.for univalent ions. Values for the divalent ions are all approximately unity like lithium. If the change in mobility were due entirely to the change in the viscosity of the solvent, the ratios in Table 11. would all have the value 1'64, the ionsH. HARTLEY AND H. R. RAIKES 395 TABLE 11. Li. Na. K. Rb. Cs. Ag. T1. F. C1. Br. I. Mg Ca Sr Ba Zn Cd moving faster in the less viscous solvent. But some other factor is con- cerned which varies from ion to ion; if it is solvation then the above ratios indicate that the ions are more solvated in methyl alcohol than in water. I t is hard to gain more precise information about solvation in methyl alcohol as experiments similar to Washburn's would be very difficult owing to the volatile and hygroscopic nature of the solvent and to the limited solubility of substances in it.Schmick's however, makes it probable that the electric field around the smaller ions is sufficiently strong to hold firmly the dipole molecules in their immediate vicinity. This accords with 3'44 3-05 3.04 2-83 3.36 3-38 - - Mobilities in Water - ..._. ___-.- Mobditics in Methyl Alcohol 5-19 5-06 4'93 5.02 5-21 4'98 - I 10 20 30 90 50 60 70 80 Atomic Number FIG. I. the general trend of recent experimental work in non-aqueous solvents, which indicates that Stokes' Law is applicable to the motion of ions,l and that the ionic radii calculated by means of it give at any rate the relative magnitudes of the ions in different solvents. Table III. gives a comparison 0'75 1.20 1.40 (0.60) (0.98) 1'02 - TABLE 111.IONIC RADII IN ANGSTROM UNITS. Methyl Crystal Water' Alcohol. Lattice. i i 2'30 3'78 1 0'72 1.79 3'27 1'01 1'22 F 1-67 CI 1'21 Br 1-18 I 1-20 - - - - - i - I - 1 - I396 MOBILITIES OF IONS IN METHYL ALCOHOL of the ionic radii in water and methyl alcohol calculated from their mobili- ties by means of Stokes' Law, together with their effective ionic radii in the crystal lattice as calculated by Wa~astjerna,~ with the exception of the values in brackets which were obtained by subtracting 0.72 from the values given by I3raggG The fact that the ionic radii in water of several of the alkali metals and halogens appear to be smaller than their effective radii in the lattice throws some doubt on the absolute values of the radii obtained by these means.The radii of all ions in methyl alcohol are greater than in water, the increase in apparent size being due presumably either to the larger solvent molecules or the greater number attached to the ion, the increase being greater for the univalent than the divalent ions. The influence of the ionic charge on the degree of solvation is shown by the fact that in both solvents all the divalent ions gave the largest radii, although the radii in the crystal lattice vary from 0.75 Angstroms for magnesium to 1-40 for barium, the greater degree of solvation of the smaller ion just compensating for the increased size of the ions of higher atomic number. The effect of size appears again clearly among the univalent ions, where the order of magnitude is the in- verse of that for the crystal lattice. Silver which is smaller than rubidium and caesium in the lattice has a smaller mobility in both solvents, while thallium which is larger in the lattice is also faster, having roughly the same mobility as rubidium and caesium. I t is worth mentioning in this connec- tion that the mobility of the perchlorate ion in methyl alcohol is 70.8 as compared with 5 1.3 for chlorine. REFERENCES. 1Nonhebel and Hartley, Phil. Mug. (rgzs), (7), 2, 729. Frazer and Hartley, Proc. Roy. SOC. (I~ZS), I ~ A , 351. 3Schmick, 2. Physik. (rgq), q, 56. 4 Ulich, Uber die Bewezlichkeit der elektroly tischen Ioncn, Berlin, 1926. Walden, Ulich, and Busch, 2. physikal. Chem. (1926), 123, 429. 6 Wasastjerna, SOC. Scient. tienn. Comm. Plays. Math. (1gz3), 38, I. GBragg, Phil. Mag. (1926), (7), 2, 258. Physica I Chemistry Laboratory, BaZZiol' CoZZege and Trinity CoZZege, Oxford.