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 5 7 1 Mr. W. R. Cooper said he was very glad that the authors had tackled this subject, concerning which so very little had been done. With regard to Table I., he could not follow from the context why the voltage should vary so much ; for example from 4-7 to 16.That was a very large varia- tion, considering that the current density was constant. Mr. D. J. Macnoughtan : Some time ago I made a few experiments upon the electrodeposition of manganese. The results obtained from manganese sulphate solution were unsatisfactory. The addition of am- monium sulphate appeared to be beneficial. This appears to be amply confirmed by the authors’ experiments and is an interesting parallel to the beneficial effect upon the deposition of nickel produced by the addition of ammonium sulphate to nickel sulphate solutions. A serious drawback to the method employed by the authors is the use of insoluble anodes which renders control of the acidity and metal content of the solution very difficult. Manganese metal of high purity is obtainable (made by the thermit process), and it is probable that with the addition of a small proportion of ammonium chloride (or other suitable chloride) to the solution, to minimise passivity, anodes of this metal would prove satis- factory and allow of continuous deposition.The view of Engemann referred to on page 570 that iron present as an impurity is the chief cause of the flaking of nickel deposits is disputed by a number of investigators to-day, and it is interesting to note that the authors found that flaking occurs in manganese deposits containing no iron. I t would be useful to know the hardness of the manganese deposits obtained, and also whether the brittleness referred to could be corrected by heat treatment. Dr. H. Borns : I have only a question which is not meant critically.The authors refer to Grube and Metzger and the &its.$ Efektrochm. as to the preparation of sheets of manganese, I mm. in thickness. Is that reference correct 3 I know the paper ; it concerns the anodic behaviour of manganese and the preparation of permanganate by means of alternating currents superposed upon direct currents. But there is no mention of the electrolytic deposition of manganese, I think. Mr. A. N. Campbell said it was stated in the paper by Foerster, in his reference to Grube and Metzger’s work, that the purity of the deposit of manganese was 99.9 to IOO per cent. and that sheets of metal I mm. thick were made by them by electrodeposition. Mr. H. J. T. Ellingham pointed out that not only had the authors found the flaking effect, which was characteristic of iron and nickel and similar metals, but there was a very curious drop of the potential to a certain value, notably baser than the equilibrium value, which remained constant for a reasonable period.That was often found as a characteristic of iron and similar metals, but could the authors give any suggestion as to the significance of it in the case of manganese. In the case of the deposition of these other metals such behaviour was usually attributed to the alloying of the metal with the hydrogen, and it was considered that this alloying is largely responsible for the flaking effect. Dr. J. N. Pring said it appeared to him that the important feature in the deposition of manganese was the high irreversible resistance, mentioned in the early part of the paper and that, of course, was a type of passivity.Judging from the analogy of other similar cases, it seemed that current5 7 2 THE ELECTRODEPOSITION OF MANGANESE density would play a very large part in the efficiency of the deposition. The results in the paper rather indicated that a low current density was favourable for the separation of manganese relatively to hydrogen and that appeared to be in line with a number of anabgous observations. At one time he himself was engaged in the investigation of the deposition of zinc fiom impure solutions, and he found that the influence of impurities such as iron and manganese could be got over by using a high current density, because in that case a very small amount of manganese or iron was de- posited relatively to the zinc, on account of the much slower reaction velocity in passing from the ionic condition to that of free metal.That was analogous to the large amount of work which had been done on this subject by Foerster.' Foerster did not actually measure the manganese, but he measured a number of other metals such as iron and nickel, and he also measured the polarisation under different conditions of current density and found that iron gave a very high polarisation and nickel still higher. I t would therefore be interesting if the results of the authors were looked into in the light of this earlier work of Foerster on other metals to see how the polarisation fits in with the values obtained. Mr. F. S . Spiers said the suggestion of Mr.Ellingham that exfoliation of iron and nickel might be due to occluded hydrogen was confirmed by some work which he did a few years ago. He found that in depositing nickel from a sulphate solution varying the acidity showed the existence of avery sharp critical value, and if the acidity was above that value there was always exfoliation. I t was also found that this exfoliation was due to some sort of stress set up in the metal which disappeared when the !metal was heated in vacuo to get rid of the hydrogen. I n copper deposition there were similar effects that could be ascribed to occluded hydrogen. I t was well known that the addition of certain colloidal substances to a copper bath of normal composition had the effect of greatly hardening the metal deposited, an effect of some technical application.Such deposits, however, were often not only hard but very brittle. When these deposits were heated to 200" or 250' C., preferably in oil, the occluded hydrogen appeared to be driven out, and while the deposits retained their toughness and hard- ness, they lost their brittleness. Had the authors found occluded hydrogen to be present in their manganese deposits? Mr. A. N. Campbell replying to the discussion said that the variation of voltage in Table I. was simply due to the impure nature of the deposit. I t had been covered more and more with basic salt which was a bad con- ductor and the voltage rose for that reason. With regard to the use of soluble anodes, if fairly pure manganese could be obtained he thought that a very good continuous process could be worked on these lines. If there were other electro-negative metals as impurities there would be the danger of the impurities going into solution subsequently depositing on the cathode and liberating hydrogen in great volume and so reducing the current efficiency. As to the hardness of the manganese, he had no actual figures ; all he could say was that it appeared to be very hard and very brittle.Regarding the effect of current density on efficiency, in the experiments described in the paper the current density was varied considerably and it was found that by increasing the current density the efficiency went up. A perfectly pure electrolyte was used and there was no question of preferential deposition of ions, except the hydrogen ion. How an impure commercial electrolyte would behave, he would not like to say. He imagined it would lZeits. Elekfrochem., 1911, 17, p. 877.DISCUSSION 573 be unsatisfactory because manganese is more electro-negative than any other common impurity. With regard to manganese dioxide forming on the cathode in the manganese chloride experiments that was really due to tetrachloride of manganese, which he had shown elsewhere to be formed at the anodes where there is plenty of acid, and that diffused to the cathode where it hydrolysed and so deposited manganese dioxide on the cathode. He believed that flaking was due to the hydrogen, as had been suggested. An endeavour had been made to overcome this difficulty by every possible means ; by very carefully purifying the electrolyte, testing for traces of iron etc., but it had not been got rid of in that way. The only constant factor was a vigorous hydrogen evolution and therefore it was thought that this must be the cause of flaking. As to hydrogen being the cause of brittle- ness he did not think that was the case because he had heated manganese in high vacuum at 300' C., and the manganese still remained brittle.