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. DISCUSSION 583 Mr. W. R. Cooper said he was sorry the author was not present as there were one or two points he wished to raise. First, with regard to the method of finding the composition of the alloys. This was found by depositing a second sample on carbon for analysis, but he felt uncertain whether one would be justified in assuming that the same alloy would be obtained in the two cases.He should have thought it would have been more satisfactory to take the analyses of the alloys as used and found on the lead ; the lead could be determined and allowed for, and the composition of the alloy found in the usual way. The footnote to Table 11. reads rather curiously : ‘‘ All current densities are given in amps. per sq, cm. and potentials in volts on the hydrogen scale,” as though there was more than one kind of volt. As to the char- acter of the curve obtained, we were accustomed to curves of this kind connecting the physical properties of an alloy with its composition, but he did not know whether we should necessarily expect the same sort of curve when it came to overvoltage, and he had been wondering whether it was simply a question of the character of the surface obtained.They all knew that overvoltage varied a great deal with the character of the surface ; possibly the curve meant that there was a maximum in roughness corresponding to the minimum of the curve. The other possibility seemed to be a variation in the rate of evolution of hydrogen owing to the amount of hydrogen absorbed. Hydrogen was absorbed very markedly in the case of iron ; he did not know to what extent it was absorbed by nickel-perhaps not so much, and therefore possibly in between there would not be such a high absorption. The overvoltage might then vary and there might be a minimum obtained in that way.He did not know why the author had taken such a high current density; he should have thought it would have been better to have worked with a low current density so as to avoid other possible effects. Mr. D. J. Macnaughton was inclined to think that the troubles encountered when using alloys made by ordinary metallurgical methods would not be greater than those experienced with alloys made by the electrochemical methods employed by the author. In Table 1. the metal content of the iron and nickel, obtained electro- lytically is stated to be IOO per cent. Such a degree of purity is doubtful, as apart from a certain considerable percentage of hydrogen, at least traces of other elements would almost certainly be present.5 g4 THE CATHODIC BEHAVIOUR OF ALLOYS The composition of the alloys determined by analysis represents the average composition of the deposit and may differ quite considerably from the composition of the surface layers. Thus during deposition using, as the author does, an insoluble anode, it is doubtful whether the hydrogen ion concentration would remain constant.This, according to the views of the author on page 579, would affect the relative proportions of iron and nickel being deposited. The crystalline structure of deposited alloys may be extremely fine or comparatively coarse according to the conditions of deposition. Such differences would considerably affect the cathodic behaviour of the deposited alloys. I t thus appears to be doubtful whether alloys prepared by deposition, as described, are superior, for the purpose in view, to alloys prepared by casting at definite temperatures, removal of the external skin by machining and subjecting if necessary to a suitable heat treatment to obtain a definite structure.The study of these electrolytic deposits is interesting because alloys or mixtures prepared metallurgically differ from alloys or mixtures prepared by the chemical or electrolytic precipitation of solutions of several metals. Those differences were the object of the researches of Tammann on isomeric alloys from liquid or solid solutions, published in 1918 in the Gottinger Nuchrichten, a publication hardly accessible over here now. One of the important points for the cathode potential and the nature of the electrolytic deposits is the relative rate of the diffusion of the two metals. The author does not appear particularly to refer to diffusion.His reference to Tammann’s paper in the Zits. f: anorg. Chm., may possibly imply a mis- understanding. I t is, I think, in accord with Tammann’s view that the deposits from solutions containing very little nickel should contain relatively more iron. Dr. J. N. Pring said the author’s results were in close accordance with the theory of Foerster in his paper in the 2eits.fiir Elekfrockrn. (1911, 17, 877), in which he found that nickel has a higher overvoltage for separation than iron ; although it had a lower potential it had got a higher overvoltage, which would be a function of the current density. This fact should account for the results of Dr.Glasstone’s work in which iron is found to be separated in relatively larger amounts than nickel. These overvoltage or passivity considerations might give a more direct explanation than the theory put forward according to which the nickel atom was supposed to have a positive catalytic effect in the separation of iron. Perhaps the point could be settled by the method employed by Foerster of using an oscillograph for measuring the potential during deposition. Such a method should also be of particular value in the measurements made with depolarisers as in the work of Le Blanc and of Reichinstein. I n this work by means of an oscillograph, a determination was made in the case of other metals and electrolytes of the three separate factors referred to in the present paper as affecting the nature of the curves shown in Fig.6. Dr. S. Glasstone (communicated reply) : I t is possible that Mr. Cooper’s suggestion that the surface of the nickel-iron alloys was rougher than that of the pure metals may account for part of the lowering in overvoltage; observation by the naked eye shows, however, that nickel and nickel-rich alloys have very smooth surfaces and yet the overvoltage is lower than at iron where the electrodeposited surface is very rough. Also it has been found that a rough iron rich alloy may have almost the same overvoltage as a smooth nickel rich one. Dr. H. Borns: I do not quite agree with the last speaker.DISCUSSION 585 From the results of Kremann and his co-workers, it is seen that the composition of an alloy does not vary very much if the material upon which it is deposited is altered ; considering the difficulties that would arise in the analysis if large proportions of lead were present, it was considered that a smaller error would be involved by depositing the alloy on carbon.A number of previous workers have shown that the composition of an alloy varies during the period of deposition, and consequently the analysis of a deposit of appreciable thickness would not give the composition of the sur- face layer. In the present work the deposits could not have been much more than 0.005 cm. thick, and hence the difference in composition be- tween the surface and the whole of the alloy could not have been very great. Further, Figs. x and 2 show that an error of a few per cent. in the determin- ation of the composition of the alloy would not affect the general nature of the results. According to the present author’s interpretation of Tammann’s views and formulae one would expect nickel in an iron rich mixture to exert only a very small protective action on the iron atoms. This may be completely counter-balanced by the large diminution in the free energy and solution pressure of the nickel, and hence the electro-deposited alloy would not necessarily contain relatively more iron than the solution.