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. 892 FIRST REPORT TO THE ATMOSPHERIC APPENDIX TO PART I. A. INFLUENCE OF DIMENSIONS OF TEST-PLATE UPON EXPERIMENTAL RESULTS. (a) Influence ufArea.-It may be observed that evidence has been ob- tained, under both indoor and open-air conditions of exposure, indicating that wetght imrement per unit areu increases slightly as the total area of the specimen decreases. This, of course, is quite intelligible in the light of the visual results which have been recorded in respect to the apparently greater amount of action on or near the edges of the specimen, and it emphasises the necessity of uniformity in the size of test-plate if comparable results are to be obtained. (b) Iiz$uence of Thickness.-In the case of copper, the influence of thick- ness of specimen has also been investigated, with, however, negative results ; i.e.plates of different thickness, but otherwise similar, have given practically identical weight-increments. Moreover, from the form of the weight-incre- ment curves obtained with this metal, it would be expected that the thick- ness of the specimen would be a factor of relatively small importance, even in the case of very prolonged exposure, and, indeed, there does not appear to be any evidence of abnormality in this respect under conditions of actual service. On the other hand, it is a matter of frequent observation that the longer life of iron articles of thick, as compared with those of thin section, is in appreciably greater proportion than the relative thickness of the materials.It is of interest to note that while other causes may also contribute, an explanation of this phenomenon is available in the shape of the weight- increment curve for iron (Fig. z j B), the significance of which, in relation to the function of the corrosion-product, was discussed in Section V.CORROSION RESEARCH COMMITTEE 893 B. OPTICAL RESULTS : EXAMPLE OF EXPERIMENTAL DATA. RESULTS OBTAINED UPON PLATES OF g o / ~ o BRASS, AT 14 DAYS’ EXPOSURE ONLY, IN ATMOSPHERE TYPE 2 (“ TANK-ROOM ”). -- After 14 Days’ Exposure. Mark. Hard or Annealed. Hard 19 Annealed i 1 Hard 1 9 Annealed 9 9 Surface Condition. (‘ Dull ” 9 9 3 9 1 9 Bright” 11 3 9 9 9 How Polished. Works L L A ” By hand Works L L A ” By hand Works (‘ A ” :rant or Back of Plate. Initial Retlec- tivity. Ro. 57 64 70 69 64‘5 63’5 52 49 80‘5 80.5 81 81‘5 85 83’5 78.5 86.0 Actual Xeflec- tivity. R f . 42-8 49’0 52’5 55‘2 48’5 49 ‘2 36.8 40.0 57‘5 62.0 59‘5 64’5 61.0 66.0 59’5 67-2 Per Cent Loss. to- Rt) 100 Ro 25-0 23’5 25.0 20’0 25 22 29 19 28-5 23 24 28 21 2 1 24 22 Mean of falues of 3ack and Front . 124.2 122‘5 +3’5 1 24’0 f.26-0 122-5 } 24’5 } 23 ‘0