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. 620 COMPRESSIBILITY OF POWDERS DISCUSSION. Dr. E. P. Perman (communicated): During the war I examined the shrinkage of a large number of cylindrical blocks of ammonium nitrate and other substances made by compression with blows. The proportion of moisture was varied in the different blocks, which were kept in a closed vessel to prevent loss or gain of moisture.The volume of the blocks was measured at definite time intervals by their mercury displacement. A beaker containing mercury was placed on the pan of a balance and counter- poised; the block was then forced under the mercury by a support from above and the balance brought again into equilibrium. The added weights represented the displaced mercury which gave at once the volume of the block. The shrinkage was most rapid at first, the greater part of the action taking place in three or four days, but it continued measureable for several weeks. This problem is different from Dr. Walker's main problem, as the shrinkage took place without external pressure (beyond that of the atmos- phere) but the forces acting must have played a part in his experiments also.Factory managers always asserted that the shrinkage was due to the work done in grinding the material. I t is possible that an amorphous and less dense form is produced by grinding or hammering,l and that this tends to revert to its original state after the grinding is over, thus causing con- traction, but my own opinion has always been that the shrinkage is due mainly to solution and crystallisation in the surface film. 'Cp. Roy, PYOC. Roy. SOL., A. 101, 509 1922.DISCUSSION 62 I My experiments gave the following results :- (I) No moisture, no shrinkage. (2) Rapid increase of shrinkage with increase of moisture. (3) The greater the solubility, the greater the shrinkage.Ammonium nitrate is very soluble and therefore shows a high rate of shrinkage. Barium sulphate and calcium carbonate showed no measure- able shrinkage. Shrinkage and caking of salts are intimately connected and, I believe, are usually brought about somewhat as follows. There is always a film of saturated solution over the surface of the powdered salt. At the points of contact the pressure will increase the solubility (with most salts); at these points, more salt dissolves and the pressure is relieved ; recrystallisation then takes place between the surfaces in contact, and caking is produced Another factor acting in a similar way is the increased solubility on rounded surfaces; this would tend to flatten the points of contact, and cause shrinkage and consolidation of the whole mass.I believe also that moisture may travel from one part to another of the crystalline mass, which is full of air spaces. This would be similar to what takes place in chemical reaction between imperfectly dried sa1ts.l The President said that Dr. Walker was to be congratulated upon determining the ratio of the velocity coefficient, and on obtaining the calculated ratio between the impact and resistance under pressure to agree so well with the observed values. A most interesting feature was the application to ammonium nitrate, which was so abnormal in several ways. I t had been shown by Dr. Lowry that moisture was necessary for the caking effect of ammonium nitrate, and that if water was not present there was no caking.He had found recently in the case of another salt- ammonium sulphate-nitrate which, it might be- remembered, was found to have caked very badly at Oppau-that if it be dried thoroughly, con- siderable pressure could be applied to it without any caking whatever; but if there was only a slight amount of moisture it caked in the same manner as ammonium nitrate did. There remained, of course, a lot of work to be done in this connection. The three different classes which these chemical compounds fell into were by no means clear from the point of view of the chemical composition of the bodies. I t was not clear, for example, why, in the case of ammonium nitrate and T.N.T., the ratio of resistance should agree so well between the calculated and observed results, whereas in the case of other bodies, such as barium nitrate and tetranitromethylaniline, or trinitrophenylmethylnitroamine as it should more properly be called, it should come outside of the calculations.I t was necessary to have other physical constants for these bodies before it would be possible to give a complete account of them, but the results Dr. Walker had got were most important and were the first, to his knowledge, to throw light on this very difficult subject, which also had a very practical bearing. Dr. E. E. Walker: With reference to the President's remarks, the classification referred to is based entirely on the behaviour of the powders towards compression. Probably a substance which belonged to Class 11. or 1x1. at ordinary temperature would be found to belong to Class I.if the temperature were raised sufficiently. Lead which anneals itself almost in- stantaneously, would certainly behave quite differently at low temperatures at which this process takes place slowly. I t is not surprising, therefore, 'Proc. Roy. Soc., A. 79, 1907, 310.622 COMPRESSIBILITY OF POWDERS that the lines of demarcation should not coincide with any recognised chemical classification. The fact that the calculated and observed values resistance to impact resistance to static load of the ratio are in fair agreement in the case of sub- stances belonging to Class I., but not in the case of substances in the other two classes, is explained adequately by the theory on which this classification is based, and is, in fact, a necessary consequence if this theory is correct. I agree on the whole with Dr. Perman’s remarks regarding the mechanism by which shrinkage is brought about, but I am of the opinion that, what- ever the mechanism, the motive force is capillary in nature. It is a well- known physical fact that, the smaller the particles are, the greater is the capillary force with which a film of moisture will draw them together. Probably in this fact is to be found the reason for the connection which undoubtedly exists between the degree of fine grinding (or “ work ”) and the amount of shrinkage observed. In my experiments the capillary force was replaced or greatly augmented by an external load, and the processes suggested by Dr. Perman are probably supplemented by simple plastic deformation, and in the earlier stages when compresson is rapid this process probably overshadows all the others.