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. 314 NOTE ON THE SORET EFFECT NOTE ON THE SORET EFFECT. BY PROFESSOR ALFRED W. PORTER. (Received I 2 th April, I g 2 7.) THE gradient of concentration of a solution corresponding to a gradient of temperature (known as the Soret effect) may be explained on the following lines.Let h be the mean free path of the solute molecules, u, the mean velocity of agitation at a layer a distance h in front of the interface, and ug the mean velocity at a distance h behind it. If the molecular concentrations at these two layers are nl n2 and if we consider one-sixth of the molecules as moving on the average in any one direction at any time, then the number crossing the in- terface per unit area per unit time in one direction is -and in the other direction is When these are equal the local concentrations remain unchanged or d(nu) = 0. But u is proportional to the square root of the absolute temperature ; hence the condition for equilibrium becomes d(nTt) = 0. The degree of accuracy with which this simple equation is obeyed is seen by examining the important sets of experimental results obtained by Mr.C. C. Tanner,l 1 Trans. Farad. SOC., 23, 91, 1927. Consider an imaginary interface in the solution. nl% 6 6A. W. PORTER Normality I n. I '1. j Tr 3'685 1 48.90 25'00 2'945 49.00 25.00 2'210 48-30 24'60 1.472 An. a,T, i. nzTz*. - ,162 64'67 65.03 - -089 38.83 38.89 - ~ 2 5 51'74 51-92 3x5 48'60 24-80 - '057 48.90 25.20 1 -*026 I '738 I n this table nl = n - +An and n2 = n + +An, n being the when the solution is at uniform temperature, 25'90 25'89 12'92 1 12'97 di. i '0057 '0035 -016 - '003 I - '00035 normality I t is seen that the constancy of nT* is very good. Even the small We defect may be allowed for by a factor, i, depending on the ionisation. should now expect inT* to be constant or d(nT4) di + 7 = 0.I di In the last column I have tabulated the values of calculated from the formula. The proportionate change in i due to the change in temperature is so small that it is a question whether it does not fall within the range of experimental error : the greatest value due to the drop of 2 0 degrees being only 7 parts per thousand. For ex- ample, for H,SO, the values calculated for 7 are very considerable. This mode of examination is safer than that adopted by Mr. Tanner because 2 0 degrees drop in temperature is too large to be treated as a dif- ferential. The gradient of temperature may be practically uniform ; it will only differ from uniformity owing to the thermal conductivity varying with the concentration and owing to the escape of heat from the sides of the vessel. On the other hand, the gradient of concentration will not be uni- form.If it be assumed that T = To + bx where b is the gradient of tem- perature, and if nT* = const. = c then 2 This simplicity does not apply to the values for all substances. di z C?J (To + 6r)B from which the average value of n is and Here T, and To are the extreme temperatures. The value of n' may be identified with the "normality" of the solution ; the values An and AT are the differences of extreme values however great. Hence we see that I An the values of - - should be inversely proportional to the geometric Z AT mean of the extreme temperatures.3 I 6 VELOCITY OF DECOMPOSITION OF NITROACETIC ACID The presence of the factor i and its difference in different cases is prob- ably the cause of the difference of the Soret effect in different cases, for if then But if then d(inT6) = o dn I dT di - + - - + + = o . n 2 T E I dT di i 2 T _ - -_ - - - dn n _ - - 0. This is presumably nearly the case with CaCl,, MgCI2, LiCI, NH4Cl, and Li2S0,. It must be added that although i is a factor introduced to allow for the complications arising from ionisation it may not be simply the van’t Hoff constant.