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. 13.118 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. CORROSION RESEARCH COMMITTEE 893 PART 11. LABORATORY EXPERIMENTS. Introductory Note.-At the time the work of the present research was commenced, the only available information relating to the mechanism of (atmospheric) oxidation of the non-ferrous metals was that due respectively to Jordis and Rosenhaupt in 1908, and to Berger in 1914. Each of these investigators had conducted experiments of a high order of accuracy with the primary object of determining whether or not the common non- ferrous metals combine with oxygen at the ordinary temperatures.The earlier workers (employing both air and oxygen as the gaseous agent) came to the conclusion that for any action to take place, with or without the pre- sence of water vapour, temperatures higher than atmospheric were necessary. Berger, on the other hand (experimenting with the metal in contact with the gas, and measuring the pressure of the latter), concluded that copper unites with dry oxygen at ordinary temperatures, although the process is not visible to the eye.Berger, however, although he dried his oxygen by the aid of potassium, and strong sulphuric acid, suggested that ‘‘ ultimate traces ” of water might be indispensable to the reaction. This worker also LOC. cit. (vide p. 849).894 FIRST REPORT TO THE ATMOSPHERIC studied the increase in the rate of oxidation with increasing temperature, and made the interesting observation that the very rapid increase observed beyond a certain temperature appeared to be directly related to the con- centration of the film of gas condensed upon the surface of the metal. Although the foregoing results are of great theoretical interest, one very important respect in which the conditions under which they were obtained, differ from those envisaged by the present research, remains to be noted.Thus, in both the investigations which have just been considered, combina- tion of the metal with oxygen was exclusively involved. Under ordinary conditions, however, and more especially, of course, in towns, the air of every-day life contains other chemically reactive constituents besides oxygen, and it is obvious that the (‘ tarnish ” which is ordinarily observed upon a metal surface may not have been produced by union with oxygen at all. Accordingly, in the present work the plan, in the first place, has con- sisted in the employment of air as occurring locally, the characteristic effects of which upon metal surfaces have already been noted in Part I.of this re- port. This has been drawn, either directly or after treatment for the removal of certain constituents, through tubes containing suitably prepared metal specimens. (The size of these specimens has been not more than I inch square so that they have readily lent themselves, where necessary, to ex- amination under the microscope.) Such a method is evidently not so academic as those just indicated, where a definite reaction was made the subject of intensive study. In the present instance, however, where not even the nature of the reactions was definitely known, some exploratory qualitative work was clearly necessary. For this purpose the above (“analytic ’7 method appeared to be more suitable, especially in view of the facility with which such experiments could be conducted concurrently with the “ field tests ” of Part I., and the obvious advantage of employing the same air in the two types of test.Nevertheless the importance of conduct- ing experiments of the purely (‘ synthetic ” type has been kept in view from the outset, and already certain tests have been made upon these lines. A description will now be given of the experimental results obtained, for which purpose it will be convenient to retain the distinction between the type of experiment which has just been indicated. A. EXPERIMENTS EMPLOYING AIR: ENTIRE, OR WITH CERTAIN CONSTITUENTS REMOVED. (I) Influence of Temperature qf MetaZ. - Two polished copper tubes were each fitted inside an ordinary Leibig’s condenser so as to take the place of the inner glass tube.Warm water (average temperature, 25OC.) was circulated through one tube, and cold water (average temperature, 15’ C.) through the other. Air (from outside the building) was in each case drawn through the annular space outside the tube at, as nearly as possible, an equal rate. The experiment was continued for a little over a month, during which time the mean temperature of the cold tube was about 10’ and that of the warm tube about zoo (C.) above the dew-point of the air passing over them. In each case the tarnishing was most severe (nearly black) at the point where the air first impinged upon the metal surface, diminishing rapidly in intensity away from this point, the exit end of the tube being apparently qui:e unaltered.7h generad degree of tarnish was much greater on fhe warm than on the coZd tube. ConcZusions.-(a) Evidently the “ tarnishing constituents ’’ are present in the air in very small proportion, and are of such a character that they areCORROSION RESEARCH COMMITTEE 895 greedily taken up by the first metal surface upon which they impinge. (b) The characteristic tarnishing phenomena upon the surface of the metal are clearly dissociated from any dew-point effect (thus confirming the conclusion reached in Part I., in respect to plates exposed in the ‘‘ base- ment ”) ; indeed they increase in intensity when the temperature is raised still further above the dew-point. I t would be interesting to trace the behaviour of the metal as the temperature is progressivZy increased to higher limits, but this has not yet been done.(2) Efect of Washing th Air through Water.-The arrangement of this experiment resembled that of the one just described, except that the air, instead of passing directly over the metal tubes, was first led through three wash-bottles containing distilled water in order to saturate it with water vapour (average temperature of saturated air = I 7’ C.). This experiment was continued for over five weeks, at the end of which time no alteration whatever was visible on the warm tube, and only a faint trace of tarnish (bluish tinge) on the cold tube. ConcZusi.ns.-(a) The ‘‘ tarnishing constituents ” are evidently very effectively removed by washing through water. (b) Air from which soluble impurities have been removed by washing, but containing water vapour almost to the point of saturation is without visible effect upon copper, even when in constant (slow) motion over the metal for considerable periods.(c) The visible effects produced by oxygen and a thin film of liquid water only are extremely small, indeed quite negligible in comparison with the effects produced by ordinary (polluted) air in the absence of liquid water. (3) E f e c t of Passing Air over Specimens Contained in a Tube.--In subsequent experiments, the specimen has been contained in a glass tube through which air has been drawn. I t is of interest to note that the general effect of drawing air over a row of‘ specimens so contained is that the first specimen receives the brunt of the attack, to the virtual exclusion of those which follow.Not only this, but the effect is very largely con- centrated upon that edge of the first specimen which is nearest the inlet end of the tube. Conchion.-This result evidently confirms that of (I) (above) as to the rapidity with which the tarnishing constituents are removed by the first (suitable) metal surface with which they come into contact. (4) Efect of Fi’dtering t h Air through Cotton- Wool.-Parallel experi- ments have been conducted by placing comparable specimens in glass tubes and drawing air over them at a similar rate (I) directly, ( 2 ) after filtering through an efficient filter of cotton wool. In the case of copper, no difference whatever was discernible up to a period of two months, the air passing through continuously at a slow rate (approximately 4 cubic foot per hour). The air was above the dew-point for the whole time, and the characteristic tarnishing effects were observed in each case.With 60/40 brass, however, the effect of #hered air at the end of a similar period had been directed almost entirely upon the alpha (copper-rich) constituent, whereas in similar experiments with unfiltered air the action had been con- centrated upon the beta constituent. Conclusions.-This latter result is of interest in connection with the “ fogging ” of the high-zinc brasses observed in the “ field tests ” (Section z of Part I.), and points to the fact that the “fogging ” is brought about by solid or liquid particles1 present in the atmosphere as distinct from en- tirely gaseous constituents.On the other hand such particles apparently This would evidently include sulphuric acid.896 FIRST REPORT TO THE ATMOSPHERIC have no effect upon copper under similar conditions ; and the experiment shows that the characteristic tarnishing effects observed upon copper above the dew-point, are brought about entirely by gaseous agents. ( 5 ) Efect of ‘ I Filtering” the Air through Granulated Silver.-Two exactly similar pieces of copper were placed each in a similar glass tube through which air could be drawn at an approximately equal rate. I n each instance the air was filtered through cotton wool before passing over the copper, but in one it was also led through a tube packed with granulated silver. The very striking result then obtained that whereas the usual pro- gression of colours developed upon the specimen over which the untreated air was passed, the other specimen (preceded by the I L silver filter ”) re- mained apparently unaltered for quite a considerable period. At the end of six weeks (when the experiment was dismantled) this specimen showed only the merest trace of discoloration, whilst the other had tarnished very severely, being, in fact, nearly black on the extreme edge.Conclusions. - Clearly the constituents of (polluted) air which bring about the characteristic colour changes on the surface of copper are just those to which the tarnishing of silver is due, the behaviour of the two metals, above the dew-point, apparently being regulated by similar causes. (It should, however, be noted that simultaneous exposure of the two metals to the same atmosphere, above the dew-point, shows that the characteristic colours develop much more quickly upon copper than upon silver.) From the known properties of silver, it is therefore evident that the visible tarnish- ing of copper under such conditions is due, not to combination with oxygen, but to reaction with sulphur compounds.( 6 ) Efect of Removal (and Additiun) of Water Yapour.-(a) Two similar pieces of copper were arranged, as before, for the drawing of air over them at approximately equal rates. In one case only a glass-wool filter preceded the copper, whilst in the other a train of drying tubes was included, consisting of several calcium chloride bulbs, and finally two tubes containing phosphorus pentoxide. On the sixth day the copper over which the normal air had passed was very appreciably tarnished, whilst that exposed to the dry air was still perfectly bright, the volume of air which had then passed over the two specimens being 49 and 5 1 cubic feet respectively.The experiment was continued for several more days until approximately 7 0 cubic feet of air had been drawn through each tube, at which time the copper in the ‘ I dry ” tube was still perfectly free from any sign of tarnish. ConcZusions.-This experiment, per se, would appear to leave no room for doubt that the presence of water vapour is definitely necessary for tarnishing to occur. Nevertheless, considering the available evidence as a whole, it was felt expedient to carry out the following additional experi- ments. (b) The arrangement here was similar to that of the previous experiment, with the addition of the following piece of apparatus between the drying train and the tube carrying the specimen ; a glass wash-bottle with stoppered funnel so that water could be admitted without the admission of air, except such as had passed over the drying materials.The experiment was carried out as before, except that from time to time a small quantity of distilled water was run into the bottle, each portion being allowed to evaporate com- pletely before a fresh addition was made. The somewhat striking result then obtained that whereas tarnishing of the blank ” proceeded in the usual manner, apparently no change was produced upon the specimen over which the treated air was passing.At the end of nine days, when 64 cubicCORROSION RESEARCH COMMITTEE 897 feet of air had passed through each tube, the one specimen displayed the usual progression of colours (from steely gray to deep orange), whilst the other showed no change beyond a mere dulling of the surface. (The total quantity of water evaporated was approximately 2 0 c.c.) Cu?tE/usions.-If the inhibition of tarnishing in (a) had been genuinely due to the removal of water vapour, it was somewhat difficult to see why the uddift'on of water vapour to the desiccated air should not have restored its capacity for tarnishing. There appeared to be only two directions in which the explanation could lie; either (I) the drying materials having actually removed whatever was responsible for tarnishing, or ( 2 ) the artificial addition of water vapour having itself in some way inhibited the process.No direct evidence relating to the former alternative has yet been obtained ; with regard to the latter, however, the following experiment was carried out. (c) Two similar copper specimens were again arranged '' in parallel," over each of which room air was drawn at the same rate. In one case only a small filter of cotton wool preceded the copper whilst in the other, a wash-bottle similar to that described above was interposed between the filter and the specimen. Water was again admitted in small quantities to the wash-bottle, whilst air was passing through. (The rate of evapora- tion was naturally considerably slower than obtained in the previous instance ; as before, however, each small edition was allowed to evaporate completely before a fresh portion was admitted.) The action apparently proceeded in a similar manner to that observed in the previous experiment, a considerable degree of tarnish having been reached upon the " blank " specimen before any appreciable change had been produced upon the other.At the end of nine days (when the blank had reached a similarly ad- vanced stage of tarnish to that described above) the specimen over which the " humidified " air had passed was seen to be in a verly early stage of colour change, this being practically confined to a narrow orange band at the extreme edge. ConcZusions.-The presence of excess of water vapour considerably reduces the rate of tarnishing of copper (in so far as this is produced by exposure to the atmosphere employed in the experiment 2).This result, surprising though it appears according to prevailing views, certainly agrees with results which have been obtained in connection with Part I. of the present report. Reverting to Experiment (6) it will now be apparent that the action of water vapour is itself quite sufficient to account for the result observed. Nevertheless, it should be pointed out that this does not necessarily ex- clude the possibility of the tarnishing constituents being removed in some way by passage over the drying agents. The results of Experiment (c) would indeed appear to throw very considerable doubt upon the necessity for the presence of water vapour, so that, in spite of the apparent con- clusiveness of Experiment (a) the question cannot be regarded as having been definitely settled.(7) Influence of" Period."-In view of the results of Part I., relating to the constancy of the form of the '' surrosion curve " for copper in an un- saturated atmosphere, it is rather remarkable that the initial rate of tarnish- ing, as judged by the eye, varies very much according to the period during which the metal is first exposed. The following figures will illustrate this point. On Friday, 6th April, an experiment was started, air being drawn The delivery tube, however, reaching only one-third of the way down the bottle. I.e. that of the '' basement." (Vi& Part I. of report.)898 FIRST REPORT TO THE ATMOSPHERIC (from outside the building) over an emeried copper specimen.On Tuesday, 10th April, this specimen was distinctly tarnished, and the total volume of air which had passed over was 15 cubic feet. I t so happened that this period coincided with a particularly cold spell of weather, the neighbouring Meteorological Office records indicating a mean temperature for the period of 43.6” F. The weather then changing quite suddenly, the specimen was repolished and replaced. For the next three days a com- paratively warm spell prevailed (the mean temperature being 56” F.) and on the Friday, when 2 2 cubic feet of air had passed over the specimen, there was no sirn of tcrmish. ConcZusions.--The above figures are given because they happen to be the only ones available which give numerical expression to a state of affairs about which no doubt is felt from the results of general observation, i.e.that a greater rate of tarnishing is associated with colder weather, the cause presumably being that more coal is then burnt, with consequent greater pollution of the atmosphere. B. EXPERIMENTS EMPLOYING CERTAIN GASES AND VAPOURS : ALONE, OR ADMIXED WITH AIR. The most obvious sulphur compound which could be held responsible for the characteristic tarnishing effects is, of course, hydrogen sulphide, and indeed such effects are frequently assumed to be caused by this compound. Whilst there can be no question of the activity of hydrogen sulphide in this respect in so far as it is actually present in the atmosphere, instances of very rapid tarnishing (as described in Part I.) have been observed under conditions where the concentration of the gas has been negligibly minute, indicating that other causes must be sought.The only other obvious constituent of polluted air which might function in this manner is, clearly, sulphur dioxide (this being the gaseous sulphur compound present in by far the largest proportion), although it is not obvious how it could, and experiment showed, indeed, that it does not bring about the familiar changes. What actually happened was that a “creamy” film was first produced upon the copper surface which subsequently passed into a permanent reddish-brown appearance. The suggestion then arose as to whether there might be among the products of combustion of coal some compound which, in conjunction with su&hur dioxide, might give rise to the characteristic tarnishing effects, and a first experiment was accordingly conducted with benzene, this being a substance known to be present in very small quantities in air polluted with such products of combustion.Two copper specimens were arranged in parallel tubes, sulphur dioxide being passed over one, and sulphur dioxide admixed with benzene vapour over the other, the rate of flow being maintained as equally as possible in the two instances. It is significant that whereas in the course of a day the sulphur dioxide alone had only given rise to the reddish-brown appear- ance already noted, the mixture had produced upon the other copper specimen a very dark-brown appearance, approaching black at the edge on which it had first impinged. The experiments had reached this stage when, very recently, a paper was published by U.R. Evans 1 describing a number of qualitative experi- 6 L The Mechanism of the So-called ‘ Dry Corrosion ’ of Metals,” Trans. Favaday SOC., 19 (1923), 201.CORROSION RESEARCH COMMITTEE 899 ments of the type now described as “ synthetic,” curiously anticipating a number of the tests with which it was proposed to proceed in the course of the present research. One of these experiments, in particular, has a very important bearing upon the present work, and is therefore quoted. Two strips of copper were suspended respectively in stoppered test- tubes, the first containing a small quantity of sulphuric acid and the second a small quantity of water, the specimen being suspended in the upper part of the vessel in each case.After leaving overnight, a slow stream of hydrogen sulphide was passed into each vessel. “ The specimen over water had become first a fine rose, then purple, then steely grey, before the specimen placed over sulphuric acid showed any change at all. After an hour, the specimen over water was dark grey, whilst the specimen held over sulphuric acid had arrived at the stage reached by the specimen over water in about a minute.” Evans concluded from this, and similar results, that the film of liquid, which he assumes to be present (adsorbed) upon the metal surface even for extremely low concentrations of water vapour, acts, together with the gas which dissolves in it, as an electrolyte, and that the tarnishing of the metal is brought about by electrochemical action between anodic and cathodic areas in the metal surface.With this view the present author has already had the opportunity of expressing his disagree- ment.l He has recently, moreover, carried out a modification of Evan’s experiment described above, the results of which appear to be of general interest. Three similar strips of copper were suspended in siniilar stoppered ‘‘ wash-bottles,” as follows : (I) over phosphorus pentoxide ; ( 2 ) over phosphorus pentoxide, the bottle containing in addition a small glass vessel placed underneath the delivery tube; (3) over a small quantity of water. In each case the delivery tube passed only two-thirds of the way down the bottle, and in ( I ) and ( 2 ) the two tubes leading into the bottle were closed with guard tubes filled with calcium chloride.After standing for a day, a little pure benzene (dried over sodium) was introduced through the delivery tube into the small vessel in ( 2 ) , the guard tube being quickly replaced. The whole were then allowed to stand for a further day. A slow stream of hydrogen sulphide was then passed into each vessel for the space of two minutes. During the passage of the gas no change at all was visible upon the specimen over phosphorus pentoxide, whilst characteristic colour changes made their appearance upon the specimen over the dry benzene. The specimen over water behaved in a very striking manner ; for an appreciable time (approximately thirty seconds) no change was visible ; the colour changes then commenced quite suddenly and proceeded with extreme rapidity ; within two minutes after discontinuing the passage of gas it had passed through a deep blue stage and had assumed a steely appearance.At three minutes, on the other hand, the “ dry ” specimen was still practically unaltered, whilst that over the benzene displayed almost the whole series of colour changes previously noted, e.g., “ steely ”-purple-deep orange-light orange. Apart from the fact that the “dry” specimen lagged very considerably behind the specimen over benzene in the development of tarnish, a true comparison of their behaviour was rendered difficult by the circumstance that the sequence of colours did not exactly correspond in the two instances. Thus, after three hours exposure, the appearance of the “dry ” specimen, whilst much the same as that shown within two minutes by the specimen Trans.Faraday Soc., 19 (rgq), 215.goo FIRST REPORT TO CORROSION RESEARCH COMMITTEE over water, could not be exactly correlated with the appearance exhibited at any stage by the specimen over benzene. On the other hand, it is worthy of note that the behaviour of the latter presented a much closer resemblance to the behaviour of the large plates in the field test (Atmos- phere Type I ) than did that of either of the other specimens. Further- more, whilst the observation of Evans as to the accelerating influence of water vapour is amply confirmed by the results of these experiments, the fact that benzene, a typical non-electrolyte, is capable of functioning in a similar manner appears to be fatal to the electrolytic explanation quoted above.With regard to the influence of water vapour it will be observed that the results of these experiments with hydrogen sulphide lead to a conclusion exactly the reverse of that previously reached in the experiments with air, where the rate of tarnishing was considerably reduced by the addition of water vapour. Clearly, therefore, the action of hydrogen sulphide does not provide an analogue of the tarnishing effects actually observed in the atmospheres under investigation. I t is evident, moreover, that much more work remains to be done in the field represented by the present experiments.’ ACKNOWLEDGMENTS. The author wishes to express his great appreciation of the advice and encouragement accorded to him by Professor H. C. H. Carpenter, F.R.S., under whose supervision the research has been conducted, and by whom many facilities have been granted. In the cordial co-operation of the staff of the Metallurgical Department of the Royal School of Mines, he has had an asset whose value he grate- fully acknowledges; he is indebted, moreover, to the staffs of the Applied Optics and Physics Departments of the Royal College of Science for their courtesy in providing facilities for carrying out the optical tests recorded in the Report. The author desires gratefully to record how very much the research owes to the sustained interest and valuable suggestions of Dr. R. S. Hutton, Director of the British Non-Ferrous Metals Research Association. With regard to the supply of materials, the research is particularly in- debted, both to the personal interest of the gentlemen indicated below, and to the generosity of the respective firms, uiz. : Mr. Thos. Bolton (Messrs. Thos. Bolton & Sons, Ltd.) ; Mr. A. Spittle (Messrs. Allen Everitt & Sons, Ltd.); Mr. H. Lancaster (Messrs. Locke, Lancaster, & W. W. & R. Johnson & Sons, Ltd.). Thanks are also due to Mr. J. C. Williams (Messrs. John Haddon & Co.), for help received in connection with the preparation of test-plates. Finally, the author gladly places on record the valuable part taken in the experimental work by Mr. L. Whitby, Research Assistant. ’A resume of the conclusions reached from these experiments will be found in the general summary of the report under the heading of ( I Copper . . . Mechanism of Tarnishing ” (p. 3).