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. PRESSURE MEASUREMENTS FOR INVESTIGAT- ING THE MUTUAL BEHAVIOUR OF ADSORBED HYDROGEN ATOMS. BY M.C. JOHNSON, M.A., DSc. Physics Department, Birmingham University. Received 23rd December, 1931. I . Introduction.-The present note is concerned with the cohesion of an adsorbed layer, or mutual attraction and repulsion between its constituent particles, rather than with their adhesion to the adsorbent. There are many important cases in which adhesion to individual atoms of the solid is small enough not to be the main factor in controlling the lateral extension or aggregation of the adsorbed layer. There is well- known experimental evidence from electron scattering and from con- densation of molecular rays that such cohesion can range from lattice-like to fluid-like behaviour of an adsorbate. The state of aggregation of adsorbed atomic hydrogen is of particular interest, since in this case the intermolecular forces cannot be deduced from knowledge of cohesion in any pure phase; we are dealing with a substance which can only maintain stable existence in the adsorbed condition, the atoms being recombined into molecules in the gaseous, liquid and solid phases.We proceed to assess some of the conditions under which it becomes possible to study HI atoms remaining in proximity to each other without forming h. The first need is for quantitative data as to closeness of this proximity in a layer adsorbed a t the gas-solid interface. 2. Density of Packing of Atoms in Layer.-If hydrogen in a perfectly sealed enclosure a t constant temperature is known to be undergoing partial dissociation by some agency confined to the gaseous phase, measurements of its fall of pressure may yield information as to the structure of an adsorbed layer, on the very probable general hypothesis that atoms can remain on surfaces they strike much longer than can molecules.For quantitative accuracy the following conditions must be fulfilled :- (a) The initial stage when no atoms exist on the surfaces exposed, and the final stage when surface density of atoms has reached its maximum for given conditions of the gaseous phase (stage of “ saturation,” but not necessarily close packing), must both be precisely determinable. For this it is necessary t o ensure a complete tracing of the various “ fatigue ” phenomena affecting rate of pressure fall. (b) It is necessary to choose an adsorbent which is not itself likely to dissociate molecules a t its surface, as for instance may occur with Tungsten and Oxygen. Vitreous surfaces are preferable to metals in this particular; for a t many metallic surfaces dissociation of the gas may have to be regarded as a consequence of, not a cause of, adsorption, in which case an atomic layer might have begun to form immediately on exposure to any stray H,, thus making impossible the fulfilment of (a).(c) No other solids beside the given adsorbing surface must make any contribution to loss or gain from the gaseous phase. I 62M. C. JOHNSON 163 ( b ) and (c) can be satisfied only if metals are completely eliminated from the adsorption vessel. This was not possible in the pioneer measure- ments of Langmuir on atomic hydrogen, since the hot filaments used as dissociating agent themselves contribute to gas losses, by processes such as those investigated later by Dillon., For this reason I investigated pressure losses in adsorbable hydrogen, devising continuous-reading methods of elucidating the factors in “ fatigue ” to comply with (a), and avoiding all metal surfaces for the sake of (b) and (c) by using as dissociating agents (i) electrodeless in- duced discharges, and (ii) the impact of mercury vapour atoms after their excitation by k2537.2 The further assumptions then needed for determination of the packing of atoms in the adsorbed layer are : - (d) Loss of N molecules from a given volume of the gaseous phase implies gain of zN atoms to the adsorbed phase over a given area.This is based on the assumptions, valid for many practical conditions, that adsorption of normal H, is negligible compared with that of H,, that excited H,’ (which is not necessarily as slow to adsorb as normal H,) is not the principal product of the disturbances (i) or (ii) proceeding in the gas, and that the temperature is not low enough for condensation of H, which possibly occurs under liquid air conditions. (e) Magnitude of the given area of adsorbing solid is equal to the geometrically measurable area. This assumption is the weakest and is certainly invalid, in the strict sense, for all surfaces except those just solidified; but if fresh glass or silica walls are thoroughly baked, but not acid treated or bombarded, this unavoidable error is probably less than for microcrystalline metals whose surface has suffered oxidation, etc.The error may reach, under optimum conditions, the order of 50 per cent. ; and this accessible surface may itself be not so much vitreous as covered with the gases which diffuse continuously out of the best baked vitreous materials. Subject to the error (e), the methods described in the papers enable initial and final stages in saturation to yield a sequence of pressure measurements fulfilling ( d ) ; these gave, under varied conditions, the following order of variation in measured maximum packing of H, atoms in the adsorbed layer, incidentally confirming monomolecular thickness and atomic state of the adsorbate :- (i) 5.9 x 1olS, 5.3 x 1015, 3-2 X 1015 per cm.,, using electrodeless discharge.(ii) 1-2 x 1015, 1.0 x 1ols per cm.2, using photosensitised dissociation. The experimental conditions of (i) must have included molecules and atoms in various states of excitation and ionisation in the gaseous phase, and it is only the degree of packing which makes it unlikely that the layer contains much else beside neutral H,. The experimental conditions of (ii) can include no ions and no excited H,’ but only normal neutral atoms, and possibly the bye-product HgH, which itself may possibly be adsorbable. In comparing the packings, it must be remembered that forces of attraction and repulsion between excited H,’ are probably different from those between normal H1,3 and hence layers formed under T. J. Dillon, Proc. Physical SOC., 41, 546, 1929. *M. C.Johnson, Proc. Roy. SOC., 123, 603, 1929; 128, 447, 1930; Proc. Kemble and Zener, Physic. Rev., 33, 512, 1929; Eisenschitz and London, Physical SOL, 4, 490, 1930. 2. Physik, 60, 523. 1930.PRESSURE MEASUREMENTS conditions (i) and (ii) would exhibit (if fluid-like), different two-dimen- sional pressures, and (if lattice-like), different spacings. 3. Conditions under which Adsorbed H, can Recombine and De- sorb as H,.-A necessary (and until lately considered sufficient) condition that an H, atom encountering another H, shall form the desorbable H,, is that a third body shall be present to receive the surplus energy in the formation of the stable molecule. This third body may be a gas molecule or a “catalytic” surface. Atoms packed to the above measured surface densities must be continually within “ collision )’ distance unless they constitute a very rigid lattice.The adsorbed layer of H, would accordingly be expected t o be in a constant state of desorp- tion by pairing of its constituents, and only capable of being maintained by a constant supply of freshly adsorbing atoms from the gaseous phase. It is possible that such a non-static maintenance of layer plays consider- able part in electrode phenomena a t the liquid-solid interface, but at the gas-solid interface under the above conditions this is not the case : for the pressure curves of all the above experiments show no rise a t room temperature, following a removal of either of the types of dissociating agent used, partial desorption only setting in a t 2 0 0 O - 3 0 0 ~ C.This con- tinued stable existence of a sheet of non-combining atoms might be explained by saying that their valency is saturated by the adsorption itself, were i t not that they recombine readily enough with fresh atoms striking them from the gaseous phase. This has been investigated by many workers beside the present author, by means of the heating of the surface during attack by fresh atoms of however small kinetic energy. Accordingly the experiments enforce a conception of HI atoms at mean distances of mutual separation only slightly exceeding the diameter of their normal Bohr orbits, and yet in mutual repulsion from each other, while still capable of attraction by and combination with similar HI impinging from the gaseous phase. 4. Cohesion Between Atoms in the Layer.-The above facts might be taken as pointing to a perfect gas state of the adsorbed atomic hydrogen, if i t were not that the further investigation yielded some indirect qual- itative evidence of cohesion between neighbouring adsorbed atoms ; this is distinct both from their common adhesion to the solid surface, and from the mutual repulsion which gives them such rigid preference for gas atoms instead of adsorbed neighbours in the far stronger attraction needed for recombination.By analysing the curves of pressure fall into terms representing condensation, spontaneous desorption, and desorption due to recombination with atoms from the gas, the last of these three processes was found to be variable, and to depend on the packing density in the layer ; * this was only explainable if, when a gas atom strikes an adsorbed atom, the latter’s neighbours can exhibit a tendency to restrain the recombination from taking place.The work done in releasing an adsorbed atom cannot, therefore, be solely against the attraction of the solid, but is also against a slight cohesion with other adsorbed atoms. Adding such data to the previous, i t becomes necessary to regard the adsorbed atoms as exhibiting both (a) a mutual repulsion preventing molecule formation among themselves, and, superposed on this, (b) a slight mutual attraction giving an incipient tendency to a state of aggregation whose packing would be loose (due to the repulsion), com- pared with the closeness of atoms within a single molecule. 4 M . C. Johnson, Proc. Roy. Soc., 132, 67, 1931.M. C. JOHNSON 165 That is, the adsorbed layer possesses both of the characteristics of a (very) imperfect gas below its critical state and tending to condense into a lattice of wide spacing. In classical theory no such combination of properties could rationally be ascribed to HI, though of course H, molecules exhibit mutual attrac- tion in very slight degree. The quantum mechanical treatment of intermolecular forces alone explains the existence of H, atoms mutually repelling at distances such as found in the above experiments: the most recent development of this to include van der Waals’ forces accounts also for the slight super- posed attraction. It is possible that such treatment of combined at- traction and repulsion nowadays considered possible to non-combining H atoms in certain states will evaluate a theoretical structure of this adsorbed layer, to which the above experiments may possibly provide, in certain features, a rough demonstration.